Classes and Objects (PHP 4)

class

A class is a collection of variables and functions working with these variables. Variables are defined by var and functions by function. A class is defined using the following syntax:

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.

The following cautionary notes are valid for PHP 4.

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).

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.

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.

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 whether the object would be named $cart, $another_cart, or something else later. Thus, you cannot write $cart->items within the Cart class itself. Instead, in order to be able to access its 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.