I'm having problem with mbstring in my Apache2.2/Win7/PHP5.3 setup, though I think it's correctly configured in my php.ini:
extension_dir = "ext"
Fatal error: Call to undefined function mb_internal_encoding() in ...
Ah, crap! PHP didn't understand the relative path. Perhaps that had something to do with me doing a manual installation...
Solution in php.ini:
extension_dir = "c:\RnD\PHP\ext"