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.

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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"