My initial thought was "surely it's just a quick hash lookup", but then require_once is probably using the canonical path for that lookup, which involves callin

My initial thought was "surely it's just a quick hash lookup", but then require_once is probably using the canonical path for that lookup, which involves calling realpath() or abspath(), at that point all bets are probably off, especially in a shared hosting environments involving things like NFS. Edit: I looked at the implementation. require_once/include_once unconditionally opens the file, whether it has been loaded already or not. Attempting to follow that path through about 10 levels of indirection (seriously), at some point the PHP implementation passes its notion of the path back to Zend, which suggests possible abspath() etc. Also somewhere in PHP's streams.c I see it unconditionally seek()ing the new file too. If nothing else, then the layers of crap I just trawled through might cause some slowness, if not some heavy library call like abspath(). I just remembered why I stopped using PHP. :)