First you need to check the active php.ini file for the PHP service. To do this, simply create an info.php file with the following content in it and access it via your browser:
<? phpinfo() ?>
It will show you the current configuration of your PHP service. Find the "Loaded Configuration File" row and copy the path to the active php.ini file. Then open this file with your favorite text editor and make the changes you want to it.