class Action
{
    public function method1($arg)
    {
        echo 'method1:'.$arg;
    }

    public function method2($arg)
    {
        echo 'method2:'.$arg;
    }
}

$act = new Action();

if (isset($_REQUEST['method']) && method_exists($act, $_REQUEST['method']))
{
    $act->{$_REQUEST['method']}('value');
}