(PHP 4, PHP 5, PHP 7)
class_exists — Checks if the class has been defined
$class_name
   [, bool $autoload = true
  ] )This function checks whether or not the given class has been defined.
class_nameThe class name. The name is matched in a case-insensitive manner.
autoloadWhether or not to call __autoload by default.
   Returns TRUE if class_name is a defined class,
   FALSE otherwise.
  
| Version | Description | 
|---|---|
| 5.0.2 | No longer returns TRUEfor defined interfaces. Use
        interface_exists(). | 
Example #1 class_exists() example
<?php
// Check that the class exists before trying to use it
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}
?>
Example #2 autoload parameter example
<?php
function __autoload($class)
{
    include($class . '.php');
    // Check to see whether the include declared the class
    if (!class_exists($class, false)) {
        trigger_error("Unable to load class: $class", E_USER_WARNING);
    }
}
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}
?>