The dev-lang/php-5.0.5-r5 ebuild (which uses php5_0-sapi.eclass) does not enable the --with-mime-magic flag when configuring PHP, which means that the Mimetype functions (mime_content_type) are missing. This is a regression from PHP 4, which *did* support this.
Won't happen, buggy and deprecated upstream. *** This bug has been marked as a duplicate of 108879 ***