If you don't want to 'hard type' in the name of the include file (as its location, and name may change, so having that hard typed in variable may break)... you could also try this method:
Inside your include file:
$myself = str_replace($_SERVER["DOCUMENT_ROOT"],"",FILE);
Then use the "$myself" variable and it will contain the relative path to the include file for use in a URL. Enjoy!