SimplePie_Misc::is_subclass_of PHP Method

    function is_subclass_of($class1, $class2)
    {
        if (func_num_args() !== 2) {
            trigger_error('Wrong parameter count for SimplePie_Misc::is_subclass_of()', E_USER_WARNING);
        } elseif (version_compare(PHP_VERSION, '5.0.3', '>=') || is_object($class1)) {
            return is_subclass_of($class1, $class2);
        } elseif (is_string($class1) && is_string($class2)) {
            if (class_exists($class1)) {
                if (class_exists($class2)) {
                    $class2 = strtolower($class2);
                    while ($class1 = strtolower(get_parent_class($class1))) {
                        if ($class1 === $class2) {
                            return true;
                        }
                    }
                }
            } else {
                trigger_error('Unknown class passed as parameter', E_USER_WARNNG);
            }
        }
        return false;
    }

 /**
  * Allows you to change which class SimplePie uses item sources.
  * Useful when you are overloading or extending SimplePie's default classes.
  *
  * @access public
  * @param string $class Name of custom class.
  * @link http://php.net/manual/en/keyword.extends.php PHP4 extends documentation
  * @link http://php.net/manual/en/language.oop5.basic.php#language.oop5.basic.extends PHP5 extends documentation
  */
 function set_source_class($class = 'SimplePie_Source')
 {
     if (SimplePie_Misc::is_subclass_of($class, 'SimplePie_Source')) {
         $this->source_class = $class;
         return true;
     }
     return false;
 }