PHP with CGI crash with extensions

When enabling PHP extensions, IIS is crashing even for a simple phpinfo() call, that is, as I uncomment extension=php_mssql.dll I immediately get:

HTTP Error 500.0 - Internal Server Error

The FastCGI process exited unexpectedly

Turns out, the issue is that the default php.ini has extension_dir = "./", the fix is to change this to:

extension_dir = "./ext"

Dont forget to restart IIS (Stop & start the server not just the Site).