Basically you should only need to enable the extension in php.ini since you’re not recompiling php. In order to do that, most commonly you would just have to remove a comment.
For PHP 4.x with Plesk, edit c:\Windows\php.ini and change this line:
with this line:
For PHP 5.x for with Plesk, edit
and replace the same line as above.
In order to make sure the new changes take effect even if the websites use PHP as an ISAPI filter, you will need to restart IIS and also make sure that if you’re running Plesk, the Plesk Control Panel service is also running after that.
You can use phpinfo(); to check if the changes you made are correct.