use $DOCUMENT_ROOT. I also had this problem, since I wanted to include a file in the include folder from different folders in the site (not just the root).
<?
include("$DOCUMENT_ROOT/include/includefile.php");
?>
Works like a charm, and the variables that have been set can also be used in the included files, this is NOT possible when using the full http path.
And ofcourse, require would work exactly the same 🙂