Ugh. As usual, more weird problems while trying to upgrade PHP on a Windows box. Are we having fun yet?

If you find that the MySQL extension won’t load, then probably a previous PHP installer has placed a version of libmysql.dll somewhere else in your path. Look in c:\WINDOWS\system32\

The problem is almost certainly the reason for the following obscure comment

Although copying libmysql.dll to the Windows system directory also works (because the system directory is by default in the system’s PATH), it’s not recommended.

in the PHP MySQL documentation.

Make sure your home PHP directory is in your path, and that its finding libmysql.dll there and only there.