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# class_exists

(PHP 4 , PHP 5)

class_exists -- Checks if the class has been defined

## Description

bool class_exists ( string class_name [, bool autoload])

> 備注: The autoload parameter was added in PHP 5

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

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`.

示範 1.autoload parameter example
`<?php function __autoload($class) { include($class . '.php'); /* Check to see it the include defined the class */ if ( !class_exists($class, false) ) { trigger_error("Unable to load class $class", E_USER_ERROR); } } if( class_exists('MyClass') ) { $myclass = new MyClass(); } ?>`

See also get_declared_classes().

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