there is no "id3" useflag in ebuild dev-lang/php to enable ID3 Functions in php. From php.net : id3 is part of PECL and can be installed using the PEAR installer. To compile PHP with id3 support, download the sourcecode, put it in php-src/ext/id3 and compile PHP using --enable-id3.
Download which source code? Plus, if it's in PECL, it should remain as PECL package, not requiring any PHP recompile. Please, clarify this request.