public static function getSingleton($class_name)
    {
        return array_key_exists($class_name, self::$_singleton) ?
                self::$_singleton[$class_name] : NULL;
    }
public static function getSingleton($class_name)
    {
        return array_key_exists($class_name, self::$_singleton) ?
                self::$_singleton[$class_name] : NULL;
    }
public static function getSingleton ( $ class_name ) { return array_key_exists ( $ class_name , self :: $ _singleton ) ? self :: $ _singleton [ $ class_name ] : NULL ; }
get a singleton instance.

@param  string $class_name
@return mixed object or NULL
get a singleton instance .