This sound like a work for
__DIR__, you can check documentation here
The directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(FILE). This directory name does not have a trailing slash unless it is the root directory. (Added in PHP 5.3.0.)
Otherwise you can still switch all your relative paths to absolute path and avoid any problem if some file are included in others.
As side note i would reccomend to not use php short tag
<?...?>, I'd rather use long tags