Hi.
I've run into the same problem, where PHP is simply unable to find extensions.
Most of the time, for me at least, it was because PHP was using the wrong php.ini config file. I usually ended up manually adding the PHP install directory to the PATH variable and running the php.ini in that directory.
Sometimes, if I'm lazy, I just copy the php.ini file into the Windows directory and run it from there. That usually works.
I've also had to use an absolute path in the 'extension_path' directive. That fixed if for me once. (Don't know why.)
Sometimes I just forget to restart apache, which is annoying, but not serious. And it seems you've already thought of that :P