mb_internal_encoding() not available though configured?

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"
extension=php_mbstring.dll

I get the following:

Fatal error: Call to undefined function mb_internal_encoding() in ...

I did a manual 开发者_运维知识库installation (by the book) using the VC6 .zip. I'm running out of ideas of what to try.

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"