src/Event.php
public static function functionExists($function_name, $only_events = false) {
        if (!$only_events && function_exists($function_name)) {
            return true;
        } else {
            return static::hasHandler($function_name);
        }
    }
public static function functionExists($function_name, $only_events = false) {
        if (!$only_events && function_exists($function_name)) {
            return true;
        } else {
            return static::hasHandler($function_name);
        }
    }
Checks if a function exists or there is a replacement event for it.

@param string $function_name The function name.
@param bool $only_events Whether or not to only check events.
@return boolean Returns `true` if the function given by `function_name` has been defined, `false` otherwise.
@see http://ca1.php.net/manual/en/function.function-exists.php