Putting code like

<!-- INCLUDEPHP test.php -->

into the overall_header.html will include that file from relative path.
If through some MODs you are in a subfolder, it will try to load subfolder/test.php.

When Debug is enabled this is no problem, but if it's disabled you get:

Warning: include(test.php) [function.include]: failed to open stream: No such file or directory in XYZ\htdocs\phpBB3\cache\tpl_prosilver_overall_header.html.php on line 92

Warning: include() [function.include]: Failed opening 'test.php' for inclusion (include_path='.;XYZ\php\pear\') in XYZ\htdocs\phpBB3\cache\tpl_prosilver_overall_header.html.php on line 92

So there are two things "wrong":
1. the message should be displayed when debug is enabled and be hidden when it is disabled.
2. there should be a way given to say whether the file is in root-path or in the relative directory.

When do you remove php from the template, it's a pain