如果使用函数头如何路径

Hello stackoverflow people. I was searching answer for a while and couldn't find it.. What if I'm editing file which is in the directory:

public_html/themes/theme_name/header.php

And i want to:

if (__FILE__ == $_SERVER['DOCUMENT_ROOT'].$_SERVER['PHP_SELF']){
    header("Location: index.php?a=error");
}

If my index.php file is in the public_html, what path should I use for this function header() because .. doesn't work?