Chapter 18. Classes and Objects (PHP 4)

Table of Contents
class
extends
Constructors
Scope Resolution Operator (::)
parent
Serializing objects - objects in sessions
The magic functions __sleep and __wakeup
References inside the constructor
Comparing objects

`class`

A class is a collection of variables and functions working with these variables. A class is defined using the following syntax:


<?php
class Cart {
    var $items;  // Items in our shopping cart

    // Add $num articles of $artnr to the cart

    function add_item($artnr, $num) {
        $this->items[$artnr] += $num;
    }

    // Take $num articles of $artnr out of the cart

    function remove_item($artnr, $num) {
        if ($this->items[$artnr] > $num) {
            $this->items[$artnr] -= $num;
            return true;
        } elseif ($this->items[$artnr] == $num) {
            unset($this->items[$artnr]);
            return true;
        } else {
            return false;
        }
    }
}
?>

This defines a class named Cart that consists of an associative array of articles in the cart and two functions to add and remove items from this cart.

Warning

You can NOT break up a class definition into multiple files. You also can NOT break a class definition into multiple PHP blocks, unless the break is within a method declaration. The following will not work:


<?php
class test {
?>
<?php
    function test() {
        print 'OK';
    }
}
?>

However, the following is allowed:


<?php
class test {
    function test() {
        ?>
        <?php
        print 'OK';
    }
}
?>

The following cautionary notes are valid for PHP 4.

Caution

The name stdClass is used internally by Zend and is reserved. You cannot have a class named stdClass in PHP.

Caution

The function names __sleep and __wakeup are magical in PHP classes. You cannot have functions with these names in any of your classes unless you want the magic functionality associated with them. See below for more information.

Caution

PHP reserves all function names starting with __ as magical. It is recommended that you do not use function names with __ in PHP unless you want some documented magic functionality.

In PHP 4, only constant initializers for var variables are allowed. To initialize variables with non-constant values, you need an initialization function which is called automatically when an object is being constructed from the class. Such a function is called a constructor (see below).


<?php
class Cart {
    /* None of these will work in PHP 4. */
    var $todays_date = date("Y-m-d");
    var $name = $firstname;
    var $owner = 'Fred ' . 'Jones';
    /* Arrays containing constant values will, though. */
    var $items = array("VCR", "TV");
}

/* This is how it should be done. */
class Cart {
    var $todays_date;
    var $name;
    var $owner;
    var $items = array("VCR", "TV");

    function Cart() {
        $this->todays_date = date("Y-m-d");
        $this->name = $GLOBALS['firstname'];
        /* etc. . . */
    }
}
?>

Classes are types, that is, they are blueprints for actual variables. You have to create a variable of the desired type with the new operator.


<?php
$cart = new Cart;
$cart->add_item("10", 1);

$another_cart = new Cart;
$another_cart->add_item("0815", 3);
?>

This creates the objects $cart and $another_cart, both of the class Cart. The function add_item() of the $cart object is being called to add 1 item of article number 10 to the $cart. 3 items of article number 0815 are being added to $another_cart.

Both, $cart and $another_cart, have functions add_item(), remove_item() and a variable items. These are distinct functions and variables. You can think of the objects as something similar to directories in a filesystem. In a filesystem you can have two different files README.TXT, as long as they are in different directories. Just like with directories where you'll have to type the full pathname in order to reach each file from the toplevel directory, you have to specify the complete name of the function you want to call: in PHP terms, the toplevel directory would be the global namespace, and the pathname separator would be ->. Thus, the names $cart->items and $another_cart->items name two different variables. Note that the variable is named $cart->items, not $cart->$items, that is, a variable name in PHP has only a single dollar sign.


<?php
// correct, single $
$cart->items = array("10" => 1); 

// invalid, because $cart->$items becomes $cart->""
$cart->$items = array("10" => 1);

// correct, but may or may not be what was intended:
// $cart->$myvar becomes $cart->items
$myvar = 'items';
$cart->$myvar = array("10" => 1);  
?>

Within a class definition, you do not know under which name the object will be accessible in your program: at the time the Cart class was written, it was unknown that the object will be named $cart or $another_cart later. Thus, you cannot write $cart->items within the Cart class itself. Instead, in order to be able to access it's own functions and variables from within a class, one can use the pseudo-variable $this which can be read as 'my own' or 'current object'. Thus, '$this->items[$artnr] += $num' can be read as 'add $num to the $artnr counter of my own items array' or 'add $num to the $artnr counter of the items array within the current object'.

Note: The $this pseudo-variable is not usually defined if the method in which it is hosted is called statically. This is not, however, a strict rule: $this is defined if a method is called statically from within another object. In this case, the value of $this is that of the calling object. This is illustrated in the following example:
<?php class A { function foo() { if (isset($this)) { echo '$this is defined ('; echo get_class($this); echo ")\n"; } else { echo "\$this is not defined.\n"; } } } class B { function bar() { A::foo(); } } $a = new A(); $a->foo(); A::foo(); $b = new B(); $b->bar(); B::bar(); ?>
The above example will output:
$this is defined (a)
$this is not defined.
$this is defined (b)
$this is not defined.

Note: There are some nice functions to handle classes and objects. You might want to take a look at the Class/Object Functions.