Frank Leonhardt's Blog

Introduction

Scripting languages are horrible. All of them. However, it is probably true to say that PHP is better than Python, so it’s not all bad. While it shares the latter’s disregard for data types, it does have classes and a full set of statements for structured flow control. Because PHP could run from within Apache, without the overhead of CGI, it gained great popularity for web sites, where “web developers” have committed unspeakable sins against computer science; but it’s not the languages’ fault.

If you’re a C++ programmer, the paragraph above will sound reasonable. If you’re taking issue with my dislike of scripting languages for software engineering, you’re reading the wrong document, because I don’t hold back.

These notes are intended as a quick crib sheet for dealing with PHP for anyone who understands basic C++ or derived languages such as Java and C#), together with Unix scripting. PHP borrows from both. It’d be hard to call yourself a programmer if you didn’t used both, but if you’ve somehow avoided the Bourne (or BASH for penguinistas) you might want to read up on the basics of these first. Whilst the control structures are borrowed from C, the variable and string syntax will be familiar from shell.

This document cuts to the chase – a quick start to PHP for programmers. It’s not a tutorial in how to program for beginners. I come neither to bury nor praise PHP. It has its uses, as do all scripting languages, and can be great fun for a quick lash-up.

Nonsense

Let’s not sugar coat this – there are aspects of PHP that don’t make a lot of sense. Here are a few you should be aware of to avoid a load of confusion:

• Overloading in PHP does not refer to overloading in anything like the normal sense of the word in computer languages. It’s actually used to talk about interpreter hooks. There is no function overloading in PHP.
• Case sensitivity is inconsistent. Variable and constant names, and labels, are case sensitive. Other things are not, including function names, class names, keywords and namespaces.
• PHP syntax isn’t necessarily backward compatible, and new syntax is added on minor versions. This document is based on PHP 7.4, but mentions some important differences with 5.x and 7.x releases.
• The meaning of “declare” has nothing to do with declaring anything; it sets interpreter options.
• Multiple inheritance isn’t supported as a matter of design, which is then circumvented using a weird non-class called a trait.
• The type declaration of “const” does not refer to an immutable object (it’s a way of declaring a manifest constant, and nothing more).
• Things like parametric polymorphism aren’t supported, but reflection is used to simulate them at run-time.

File format

PHP interprets lines from a text file:

#!/usr/local/bin/php
<!--?php
print (“Hello World\n”);
?-->

Like any other script language, it helps to put the name of the interpreter in the first line of the file although if it’s being run through Apache you won’t need to.

#!/usr/local/bin/php

If you’re using Linux rather than Unix this may be at /bin/php, of course.
Next there is the <!–?php … ?–> business. The PHP interpreter was intended to allow PHP code to be embedded in an HTML tag, so this is the opening and closing of a tag to get it to take notice of the stream.

On later (current) versions you can dispense with the HTML comments and wrap the file in <?php … ?>

And in the middle, there’s our print statement. And it is actually a statement; not a function. I just put the brackets around it because I could. It prints a single string.

Note the semicolon on the end – like ‘C’, line breaks are irrelevant and statements are ended by a ‘;’

PHP also includes the echo statement, which takes multiple arguments like echo in the shell:

echo "foo", "bar";

Having two ways of doing the same thing is all part of the fun on PHP.

Comments

As you might expect from the offspring of C++ and Bourne shell, you can use both types of comment. C++ // and /* */, as well as #. For the sake of sanity, I’ll stick with // in this document, but it’s a matter of taste which you prefer.

Constants

Literal constants follow the conventions you’re used to, with the exception of string literals, which follow normal shell convetions. That’s to say that they may be quoted with either a single (‘) or double (“) quote. A single quote does not produce an integer character code. This is covered in more detail in the Strings section.

Other literal constants are as you’d expect – 123 is a cardinal; 1.23 is a float, and True is a Boolean.

Manifest constants in PHP are, by convention, declared in UPPER CASE, although it doesn’t matter to the language. There are two ways to do it:

const PI = 3.141592;
define ("Euler", 2.71828);

If you redefine a constant PHP will ignore you, silently. In other words, the first definition persists even though this is irrational.

At first sight, define() may look like a pre-processor option, but in an interpreted language this doesn’t really make sense. However, in combination with a conditional, you can use define() to create a constant with a value determined at run-time. The const modifier is a later addition to PHP and works like a compile-time definition (but with no error-checking). It has the advantage that it’s sensitive to namespaces and you’ll probably prefer it going forward. Until PHP 5.3 it only worked in class scope, but this restriction has gone.

It’s widely believed that the initialiser in a const declaration can’t be an expression. It can (since PHP 5.6); it just has to be a constant expression – e.g. const RPI = 1/PI;

Operators

PHP implements the usual operators with the usual precedence, with a few extras.

‘.’ is the string concatenation operator.

‘**’ is the exponential operator (from PHP 5.6 onward).

!== and === differ from != and == in that they must be identical – no type conversions are performed so the expressions being compared must be of the same type to be the same value.

<> is an alternative to !=, although bizarrely it has a precedence one below.

Only a maniac would use this feature in coding.
Comparison operators also work on arrays, and plus (‘+’) can be used to concatenate arrays (i.e. a union operator).

And as of PHP 7, there’s the <=> operator. It returns zero if both arguments are the same, -1 if the left argument is less than the right, and +1 if it’s greater. This is similar to the strcmp() function in the POSIX library, but the thought process that came up with it being a good idea as an operator is baffling.

Variables and Types

All variables are prefixed with a $, whether they appear in an lvalue and as an rvalue (unlike shell). But they don’t have a type.

$a=2;
$b=$a*10;
$a='Twenty: ';
echo $a ,$b;

As you can see, I stored an integer in $a, and then stored a string in it. Variables do have dynamic typing, although at any one time they are of a specific type. There’s a function to return the current type as a string:

$myint=123;
$mystr="A string";
$myreal=3.14159;
$mybool=true;
echo gettype($myint),": ",$myint,"\n";
echo gettype($mystr),": ",$mystr,"\n";
echo gettype($myreal),": ",$myreal,"\n";
echo gettype($mybool),": ",$mybool,"\n";
Output:
integer: 123
string: A string
double: 3.14159
boolean: 1

There are several functions like is_int() for other types, and they return true or false as you’d expect. And speaking of Boolean values, true is the integer 1, whereas false is an empty string. For conditionals, 0 or non-zero work as false or true too; as do empty or non-empty strings.

$a = (3 > 5);
$b = (3 < 5);
print ("a=$a, b=$b");

Output: a=, b=1 

Variables in PHP are self-creating on first use, and that includes using them as part of an rvalue. The following nonsense is legal, with $b printing as an empty string. If you’re lucky you’ll get a notice-level warning to alert you to the fact you’ve messed up, but this isn’t guaranteed by any means.

$a="Hello";
echo $a, $b;

Output: Hello 

Strings

Numeric expressions work as you’d expect. Strings can be concatenated using the ‘.’ operator (which you should separate with a space. Expression types are cast in a sensible way, so “Testing” . 123; results in a string containing “Testing123”.

$name='Mary';
$day='Monday';
$age=6;
print ("It will be $name's Birthday next $day and she will be $age.\n");

Output: It will be Mary's Birthday next Monday and she will be 6.

As you can see I haven’t quoted the single quote – there’s no need. The usual conventions apply, so to get a double quote you’d need \”. If your variable name isn’t obvious because it has printable characters immediately before or after, put it in curly brackets thus:

{$name}

PHP has dozens of string functions for you to look up, including everyone’s favourite strlen(). Some take the form str_xxx() and some are named strxxx(), for no good reason I can see except for PHP being bonkers.

Arrays

PHP doesn’t go for types, so arrays are also un-typed; and can be of mixed type values. Arrays are also dynamic. In fact they’re really lists, but PHP heads called them arrays anyway.
You can create arrays with an array() function, but the simple way is self-explanatory assuming you’re using a relatively recent version of PHP, as below:

$a1 = []; // Empty array
$a2 = [1,2,3,"Tom","Dick","Harry"]; // Initialised with stuff

If you’re using ancient PHP you could get the equivalent using:

$a1=array();
$a2=array(1,2,3,"Tom","Dick","Harry");

In the above, $a2[3] would be “Tom”, as you might expect. However, PHP arrays only default to zero based integer addressing. The index can be anything else, like a string. (Associative array is a name that may sprint to mind). The full syntax for initialisation is:

 $a3 = [ "red" => "rouge", 
         "blue" => "bleu",
         "green" => "vert",
         "yellow" => "jaune" ];

And $a3[“green”] would return “vert”. You can mix integer indexes in with the strings, and if you don’t specify an index for an element it defaults to the next integer index. That’s to say, the last integer index used plus one. For example the following is perfectly legal, even if it isn’t sane:

$a3 = [ "red" => "rouge",
        6 => "bleu",
        "green" => "vert",
        "jaune" ];

print ("a3[7] = $a3[7] \n");


 
Output: a3[7] = jaune 

Note that this wouldn’t be $a3[8] or $a3[3]; both of which you might expect. This is because the second element had its index set to 6, the third element was given a key and the eight element defaulted to the next integer – 6+1 = 7. $a3[0] won’t get your “rouge”, as it’s got a key instead.

If you’re using an associative array (or integer indexed one), the risk is that PHP will bomb out if it can’t find the index/key. You can avoid this using the array_key_exists() function. It returns a bool and takes two arguments, the first being the key and the second the array. There are a number of array functions similar to those in the STL – in_array(), array_push(), array_pop(), count() sort(), and so on.

To add elements to an array this syntax is:

$a2[] = "Another";
$a3["purple"] = "violet";

The first adds an item indexed by the next available integer index. The second form adds a key/value pair. And yes, if the key or index already exists you’ll over-write the current value with your new one.
To remove an item from an array, you need the unset() function. It basically takes one or more variables or array locations as parameters and removes them, as if they’ve never been set

unset ($a3[6], $a3["green"); // Blue and green should never be seen.

Multi-dimensional array access is the same as for ‘C’ – i.e. as many indexes as you need each in their own square brackets

Flow Control

I hope you had fun with variables and strings. Now we come to some good news. The flow control structures in PHP are the same as ‘C’, Java and C#. Including blocking statements together with curly brackets.

All of the following are valid and work exactly as you’d expect.

if (conditional) statement;

if (conditional) { statement; statement; … } 

do statement; while (conditional);
 
do statement; while (conditional); 

while (conditional) statement;

for (initialiser; conditional; increment) statement;

 switch (value) // Value can be any type
{
    case 1:
        statement;
        break;
    case 2:
        statement;

    default: 
        statement;
}
 
goto label; 

There’s also an “enhanced” for loop as in Java, C# or shell script, or the C++ STL:

foreach (array as item) statement;

Basically, each time through the loop the variable “item” is replaced with the next element in “array”.

foreach ($a3 as $element)
    print ("The item is $element\n");

Whilst Java omits goto; PHP doesn’t, in common with C, C++ and C#.

In complex code a goto statement can simplify flow control considerably, although it’s hard to come up with a trivial example where it makes sense. Here follows a trivial example where it doesn’t make sense, but you get the idea. Note the label ends in a colon (“:”).

goto mylabel;
print ("This will NOT be printed\n");
mylabel:
print ("Output will start here\n");

Finally, “break” and “continue” work as expected in loops.

In spite of having a full set of structured statements, many PHP developers seem to like using exceptions to implement program flow. More on this later.

Functions/Methods

PHP functions are a simplified version on C/C++/Java/C# syntax. Note that functions are called methods in Java.

So here’s a function:

function square ( $x )
{
    return $x * $x;
}

print("2 squared is ".square(2)."\n");

Output: 2 squared is 4 

And that’s about it. The keyword “function” tells the interpreter it’s about to get a function definition.

As PHP isn’t typed, there’s no need to worry about specifying a return type, or parameter types. This is probably one of the worst aspects of K&R ‘C’, which was corrected with function prototypes in the ANSI specification. PHP made the same initial mistake, and implemented something similar to function prototypes in PHP version 5 onwards where they were known as type hints. From version 7 they’re called type declarations and have been extended.

Even with type hints, function name overloading isn’t possible; there is no compile-time polymorphism as there is no compilation. However, introspection and reflection are supported in PHP and this is used for un-checked dynamic polymorphic constructs.

The syntax for declaring an (optional) function return type is different to other languages (PHP 7 onward):

function sum($x, $y): float
{
    return $x + $y;
}
print("The type of sum() = ".gettype(sum(2,3))."\n");

Output: The type of sum() = double 

As you can see, PHP also “rounds up” floats to doubles. The value of return types may be a bit limited, as most of the time dynamic typing will silently convert it to whatever seems right anyway.

In addition to class names, the return type of a function can be one of self (instance of the class the method was defined on), array or callable. From PHP 7 onward you can have book, float, int and string. In 7.2 return types of iterable and object have been added.

The syntax for arguments is the same as C/C++/Java/C#. If you want to use argument type checking, you can write it as:

function square ( int $x )
…

It’s worth noting that without the optional type checking (which is normal for PHP), a function will return something random if you give it unexpected arguments. There’s a convention that a NULL will be returned in such circumstances, but it’s only a convention.

You can pass by reference by prefixing the variable name with an ampersand “&”, (C++ style).

function square ( &$x )
{
    $x = $x*$x;
}

If a function returns a reference, prefix its name with &.

function &add_tax (&$x)
{
    $x *= 1.2;
    return $x;
}

You can also have default parameter in the same way as C++

function printwhen ( $product, $when="sometime")
{
    print ("Your $product will be ready $when.\n");
}

A variable arguments list is accomplished using … as prefix to a variable, which effectively becomes an array of the passed values.

function total (...$numbers)
{
    $tot = 0;
    foreach ($numbers as $num)
    $tot+=$num;
    print "The total is $tot\n";
}

 
total (3,1,4,1,5);
Output: The total is 14 

And for one final piece of craziness you can call a function by having its name stored in a string.

function hi()
{
    print "Hello\n";
}
$greeting ="hi";
$greeting();

This does not make it a first-class function, or even a pointer to a function. It’s a recipe for disaster.

If you’re wondering about void functions, there’s no such thing, but if you don’t hit a return statement PHP will return a NULL.

Lambda (closure; anonymous) functions are supported:

function myfunction ($a1, $f1)
{
    print ("a1=$a1\n");
    $f1 (456);
}
myfunction (123, function ($a) { print ("a=$a\n"); } ); 

This will output

a1=123
a=456

You can make the lambda inherit variables from the local scope with use():

$x = 1; $y = 2;
myfunction (123, function ($a) use ($x, $y) { print ("a=$a, x=$x, y=$y\n"); } );

Output:

a1=123
a=456, x=1, y=2

Just keep reading it until in makes sense.

Classes

The class/object model is pretty much a subset of the same as C++, which has been borrowed by Java and C#. If you’re not used to C++ terminology:

• method = member function
• property = member variable = attribute

You define a class using the class keyword followed by the name. The modifiers const, static, public, private and protected have the usual meaning. By default everything is private, but this can be overridden on an element-by-element basis. Unlike C++, you can’t use public:, protected: or private: as labels to change subsequent declarations. There’s no such thing as a “struct” in PHP (which is a class where members default to public, if you’re coming from Java or C#), or a union.

The final keyword can be applied to a class or method, and prevents the class being used as a base for a derived class, or the method being overridden. It’s not the same as Java when it comes to class constants; in PHP use the keyword “const” instead.

A constructor is defined as “public function __construct() …”, with optional arguments. Remember, function name overloading isn’t supported. A class’ destructor is a function named __destruct().

Functions starting with __ (two underscores) are “magic” in PHP. Additional magic functions defined include __call(), __callStatic(), __get(), __set(), __isset(), __unset(), __sleep(), __wakeup(), __toString(), __invoke(), __set_state(), __clone() and __debugInfo(), and might best be described as hooks within a class that get called by the interpreter in particular circumstances. See a PHP reference for further details.

The pseudo-variable $this has the same meaning as other languages. There’s also a keyword “self” (no $) that can be used to refer to the class rather than this particular instantiation (object of class), and “parent”, to refer to the parent class where inheritance is used.

To reference an element within an object of a class, use the arrow operator (“->”), as if it’s a structure pointer dereference in C++. In languages without pointers, like Java and C#, you always use the “.” operator to specify an element, whatever the underlying reality. In ‘C’, the choice of . and -> makes a difference. In PHP, you always use “->”; at least you don’t have to think about it.

So here’s an example, which should be self-explanatory, but I’ll explain it afterwards anyway:

class myClass
{
    public $myVar = "My default value";
 
    function printme()
    {
        print("myVar = " . $this->myVar . "\n");
    }
    function printhello()
    {
        print("Hello!\n");
    }
}

$myObject = new myClass;
$myObject->printme();
 
$myObject->myVar = "New value";
$myObject->printme();
myClass::printhello(); 

Output:

myVar = My default value
myVar = New value
Hello!

First off, note that the attribute $myVar is declared with at $ but this isn’t used when it’s referenced as an element. I’ve made it public so it can be referenced from outside the class.

I haven’t made the printme() method public; it is by default! Inside it I’ve used $this->myVar to get at the attribute. Without the $this-> it won’t be in scope. In other words, properties are NOT in scope of methods.

PHP supports the scope resolution operator “::”, which is used above for the sake of simple demonstration. printhello() is the same as a Class method in Java. Note that myClass::printme(); would fail, because no instantiated object exists for $this to refer to.

The scope resolution becomes important when you use derived classes.

class myChild extends myClass
{
public $childVar = "I'm only a kid"; 

function printme()
    {
        myClass::printme();
        print("myVar another way = " . $this->myVar . "\n");
        print("childVar = " . $this->childVar . "\n");
    }
}
 
$myChildObject = new myChild;
$myChildObject->myVar = "I'm the parent object.";
$myChildObject->printme(); 

What won’t work is if you have properties of the same name in both the base and derived class. With the auto-creating variables in PHP, “declaring” another myVar in myChild will make it indistinguishable from the same property in myClass. No warning. Just confusion; so take care you never do it. If BOTH properties are private, that’s fine.

class myClass
{
private $myVar = "I'm the parent";
    function printme()
    {
        print("myVar in base = " . $this->myVar . "\n");
    }
}

class myChild extends myClass
{
private $myVar = "I'm only a kid";
    function printme()
    {
        myClass::printme();
        print("myVar in child = " . $this->myVar . "\n");
    }
}
 
$myObject = new myChild;
$myObject->printme(); 

Output:

myVar in base = I'm the parent
myVar in child = I'm only a kid

To access a static class property or function (aka attribute or method) prefix it with the class name and the scope resolution operator (not a dot!)
Interface and Abstract Classes

In C++ an interface class contains one or more pure virtual functions (abstract methods) and nothing else. It’s a special case of an abstract class. However, because a class is a class, it’s treated as any other. Multiple inherence lets a new class inherit as from as many parents as it likes; interface or not.

In Java and C#, with single inheritance, multiple interface classes can also be inherited. However, they’re up-front about an interface class, with its own special keyword. PHP shares this, as well as Java’s “implements” keyword when specifying an interface in a class declaration.

Here’s a simple example of an abstract class:

abstract class AbstractClass
{
    // Force an extending class to define this method
    abstract protected function printme();

    // Example method everything will inherent

    public function printHello ()
    {
        print "Hello World\n";
    }
}

class ConcreteClass extends AbstractClass
{
    public function printme()
    {
        parent::printHello();
        print("I’m concrete\n");
    }
}

$rubble = new ConcreteClass;
$rubble->printme(); 

Output:

Hello World
I’m concrete

If you’re having an interface class (abstract class consisting of only pure virtual functions), in PHP you say so. A concrete class can inherit multiple interfaces (using a comma separated list), even if it can only extend one base class.

interface iPrintable
{
    public function printme();
}

interface iFriendly
{
    public function printHello();
}

class MyClass implements iPrintable, iFriendly
{
    public function printme()
    {
        print("I’m concrete\n");
    }

    public function printHello()
    {
    print("Hi there!\n");
    }
}

$myObject = new MyClass;
$myObject->printHello();
$myObject->printme(); 

Output:

Hi there!
I’m concrete

I’ve previously mentioned that PHP doesn’t support multiple inheritance. To get around this PHP does include something called a trait (from PHP 5.4). If the answer to your problem involves using traits, you’re probably doing something wrong., but you’ll need to be aware of them.

class MyClass
{
    public function printHello()
    {
        print ("Hello");
    }
}

trait PrintWorld
{
    public function printWorld()
    {
        print ( " World!\n");
    }
}
 
class Greeting extends MyClass
{
    use PrintWorld;
}

$myObject = new Greeting();
$myObject->printHello();
$myObject->printWorld();

 
Output: Hello World!  

As you can see in the code, MyClass and Greeting are both classes. Greeting extends MyClass but uses a trait called PrintWorld. As a result, Greeting inherits the printHello() method from MyClass and the printWorld() method from the PrintWorld trait. You can use as many traits as you like, but there are, unsurprisingly, more limited than a full class. If they have a use, it’s to lump a load of functions together in a convenient place. If you use traits for anything tricky, and need to resolve clashes and work out precedence, you’ll have to read the full manual; and when you do remember it was your fault for getting into the mess in the first place, because I warned you.

Namespace

PHP does support namespaces from PHP 5.3 onward, but it’s clunky. The keyword “namespace” followed by your chosen name must be the first thing in the source file, other than the opening tag (<?php). This is very strict, meaning that you can’t have anything before the tag, including initial html.

Good:

 <?php
 namespace Example;

Fatal Error:

 <?php
 namespace Example;

Namespaces are hierarchical, and the path can be separated using a backslash (‘\’) – not the scope resolution operator. You can access anything public in a different namespace if you know how to get to it:

namespace test/junk/foo;

\test\junk\somefunction();

You can import classes from another namespace with the “use [… as …]” keyword:

 use \test\junk\myClass as hisClass;

From PHP 5.6 you can also have “use const …” and “use function …”

Reflection and Introspection

Because PHP is interpreted it’s technically possible to mess around with objects at runtime; not the object’s members but the languages’s notion of an object. This can lead you many interesting and amusing problems, but is sometimes the least-worst option for dealing with the fluid object typing inherent in the language.

Let’s be clear here – in assembly language its possible to use self-modifying code. It was clever, and when you had 1K of core to play with, its use was justified. It’s also possible in ‘C’ – you can find the address of a function easily enough and modify the op-codes. However, it’s pretty obvious to anyone that this is a path to madness. And with modern compiler optimisation, wholly unnecessary.

So where you see introspection used sensibly is on checking the arguments to a function. To PHP, every object can look like any other, which appears to be a good reason why parametric polymorphism can’t happen. However, if you write a single function that examines the objects actually passed and acts accordingly.

If you want to read more about this, as ever, the PHP documentation is the place yo go. However, here’s a quick example to give you the idea of how to get started.

class Foo
 {
         private $my_private_var;
         public $my_public_var;
         protected $my_protected_var;
     function print_my_name ()
     {
             echo "My name is Foo\n";
             $this->my_private_var = "I'm Private";
             $this->my_public_var = "I'm Public";
             $this->my_protected_var = "I'm Protected";
     }
 }

 // Instantiate a Foo, check it's working and set it's variables.
 $test = new Foo;
 $test->print_my_name();
 
 // These two functions get the type of a variable
 // (in this case it'll be an object) and the class
 // name of the object, so your function can decide what
 // to do with it.
 echo "gettype() returns " . gettype($test) . "\n";
 echo "get_class() returns " . get_class($test) . "\n";

 // As these next functions are returning arrays, we're going to
 // use the print_r() function to print them in readable format
 // from now on. And here's where it starts to get silly.

 echo "Output of get_object_vars()\n";
 print_r(get_object_vars ($test));

 echo "\nOutput of get_class_vars()\n";
 print_r(get_class_vars ('Foo'));

 echo "\nOutput of get_class_methods()\n";
 print_r(get_class_methods ('Foo'));

 echo "\nOutput after casting object to array\n";
 print_r((array)$test);

 echo "\nvar_dump() after casting to an array\n";
 var_dump((array)$test);
 
// And now for something really horrible:

 $a=(array)$test;
 $a["Var_no_one_knows_about"] = "I've got a bad feeling about this.";
 $test=$a;
 
 echo "\nThe Foo object after we've hacked it.\n"; 
 print_r((array)$test);

Output

 My name is Foo
 gettype() returns object
 get_class() returns Foo
 Output of get_object_vars()
 Array
 (
     [my_public_var] => I'm Public
 )
 Output of get_class_vars()
 Array
 (
     [my_public_var] =>
 )
 Output of get_class_methods()
 Array
 (
     [0] => print_my_name
 )
 Output after casting object to array
 Array
 (
     [Foomy_private_var] => I'm Private
     [my_public_var] => I'm Public
     [*my_protected_var] => I'm Protected
 )
 var_dump() after casting to an array
 array(3) {
   ["Foomy_private_var"]=> string(11) "I'm Private"
   ["my_public_var"]=> string(10) "I'm Public"
   ["*my_protected_var"]=> string(13) "I'm Protected"
 }
 The Foo object after we've hacked it.
 Array
 (
     [Foomy_private_var] => I'm Private
     [my_public_var] => I'm Public
     [*my_protected_var] => I'm Protected
     [Var_no_one_knows_about] => I've got a bad feeling about this.
 )

Debugging

There are many PHP functions intended for debugging, but here are a few tips to start you off.

print_r() – Pretty print a variable or array.

var_dump() – As above, but with more information (particularly its type)

debug_zval_dump() – As above, but with reference counts for debugging the PHP interpreter.

Here’s a quick example.

$a = ["One","Two","Three"];
print_r($a); 
var_dump($a);

Output

 Array
 (
     [0] => One
     [1] => Two
     [2] => Three
 )
 array(3) {
   [0]=> string(3) "One"
   [1]=> string(3) "Two"
   [2]=> string(5) "Three"
 }

One very useful feature of the var_dump() function is that when it’s used for a class it can be overridden by the special “__debugInfo()” member function, which can output the contents in any way you see fit. The default is to dump the lot.

Because debugging is something you’re likely to want to do without wanting to read the whole PHP manual, I’ll go into a bit more detail. These functions will write to the standard output (or HTML stream if you’re using it for a web site). To catch teh output and write it to a file use something like this:

// First we capture the output into an output buffer
// and store the result in the string.
ob_start();
var_dump($a);
$stuff = ob_get_clean();

// Then we append it to a text file of our choice
$fp = fopen("/var/log/mylog.log", "a");
fwrite($fp, $stuff);
fclose($fp);

It’s easier to capture print_r() as it has a default final parameter; a flag to return the output as a string rather than printing it. In this example I’m using the special variable $GLOBALS to dump every global variable.

$stuff = print_r($GLOBALS,true);

Miscellanies

Running shell commands

PHP supports the back-tick operator in the same way as the Bourne shell. Anything between two back-ticks (‘`’) will be passed to the shell, and stdout will replace the whole lot in the interpreted line.

declare()

There’s a keyword “declare” that is akin to a compiler #pragma. It sets various options for the interpreter from this point onward; or for the following block. It doesn’t actually declare anything.

Examples:

 declare (strict_types=1);

This will actually enable type checking on function arguments, assuming someone’s bothered to define types in the first place.

 declare(encoding='ISO-8859-1')
 {
     …
 }

The code between the { … } is encoded using ‘ISO-8859-1’ (i.e. 8-bit extended ASCII, not Unicode).

include/require

These statements simply include an external file into the interpreter stream at this point.

 include “something.php”;

Require is similar, except it’ll halt execution with a fatal error if the file isn’t found. There are special versions, include_once and require_once, which will refrain from including the named file if it’s been included before.

@ Error control operator

Being an interpreted language, you won’t be bothered by any compile-time errors drawing attention to mistakes in your code. If you want to prevent pesky run-time errors to, you can prefix any expression you like with a commercial at (‘@’), and it’ll suppress any errors or warnings that would otherwise highlight your mistakes. This includes fatal errors, so your script will just stop dead, causing the user to utter many interesting terms of endearment about you when they find out.

As far as I can make out, the @ operator technically sets the run-time error reporting level to zero (that which is returned by error_reporting ()). Any error handlers you’ve set will still be called.

Auto loading functions

PHP programs can become a mess with a lot of includes at the beginning of each script file in order to import a large number of classes. As a result its quite common to use a mechanism called auto-loading. Prior to PHP 8 this was done using a __autoload function() but newer versions use spl_autoload_register() instead.

With __autoload() (two underscores) you basically defined the function and it will be called if some of your code fails with a “Class not found” error. The function takes a class name as its parameter (the one that caused the error) and is likely to include that class from wherever you have it stashed.

This mechanism does, at least, only load classes when they’re first called.

function __autoload($ClassName)
{
    include($ClassName.".php");
}

With PHP 8 it’s a bit more complex. __autoload() has been removed, so you have to use spl_autoload_register() – although you can use this to call an __autoload() from your old code. The difference is that __autoload() has no special meaning any more but it’s still a valid function.

As its name might suggest, it registers a load of autoload functions that the system can use as required.

The function takes the name of an autoload function (which defaults to spl_autoload()) as its first parameter. Two more optional boolean flags are passed – the first can be set to false if you don’t want it to throw an exception, and the second can be set to true if you want the function registered at the front of the list rather than the end.

You’d probably be best reading the full documentation if you come across something that isn’t obvious in the code.

Iteration

Since Version 5, PHP has supported external iterators for arrays objects. There are a couple of dozen predefined, and each of which has a similar number of methods. You’ll have to read the PHP documentation for a comprehensive list. Unfortunately the documentation is lacking for many of them.

Like other languages, an Iterator is an interface that an object may implement. This includes the following methods:

current() – return element pointed to by the cursor.
key() – return the key pointed to by the cursor.
next() – move the cursor on by one.
rewind() move the cursor to the first element.
valid () – return true if the cursor is pointing to a valid element (e.g. it hasn’t gone off the end).

Exception Handling

There really needs to be a section on exception handling. It’s very similar to the Java model, but the way it’s used makes me nauseous and I can’t bring myself to write about it right now.