I'm not sure if you post is unclear, or my brain is. Probably it's the latter ... I'm often that way.
Based on my reading of your post, I'll say this:
<?php
//one.php
include 'two.php';
?>
<?php
two.php
echo $_SERVER['PHP_SELF'];
?>
If "one.php" includes "two.php", and "two.php" refers to the PHP_SELF variable, it's referring to the name/relative URL of "one.php".....
OTOH, if you're using GET to add content, why isn't it in the code you posted? <?php echo $_SERVER['PHP_SELF']."?include=foofile.php"; ?> ??