What I did:
Function in remotefile.inc:

function listusers(){
  echo "Blah <a href='$PHP_SELF?file=mainpage'>Link</a> blah">
}

Then require file in my main.php file.

require_once("remotefile.inc");

And call the function

What I expect is for the output to be:

Blah <a href='main.php?file=mainpage'>Link</a> blah

Which infact it is.

However, I also get a low level error message saying PHP_SELF is blank.

Current solution for problem:

function listusers(){
  $PHP_SELF = "";
  echo "Blah <a href='$PHP_SELF?file=mainpage'>Link</a> blah">
}