the class_exists(str $class) function has two forms:

class_exists(str $class) and class_exists (str $class, bool $use_autoload).

If the second value is not false, any autoload routines (such as autoload module's, the zend (Zend Framework) module, or what have you) will attempt to load a given class from the filesystem.

This isn't productive is you are simply searching for classes that are native to the PHP core such as PHPmailer or ReflectionClass in afmphp or messaging, or if you are searching for classes that you intend to only be available if you have included their source files manually.

So, unless you intend to trigger an autoloading mechanic, always use class_exists ($class_name, FALSE).