PHP Manual: class

Description

bool class_exists ( string class_name [, bool autoload] )

This function returns TRUE if the class given by class_name has been defined, FALSE otherwise.

Example 1. class_exists() example

<?php
// Check the class exists before trying to use it
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}

?>

class_exists() will attempt to call __autoload by default, if you don't want class_exists() to call __autoload, you can set the parameter autoload to FALSE.

Example 2. autoload parameter example

<?php
function __autoload($class)
{
    include($class . '.php');

    // Check to see if the include declared the class
    if (!class_exists($class, false)) {
        trigger_error("Unable to load class: $class", E_USER_WARNING);
    }
}
    
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}

?>

Note: The autoload parameter was added in PHP 5