How to add php.ini Directive on Plesk

1. Open your domain in the Plesk Control Panel
2. Click the Subscription
3. Click Open in Control Panel
4. Check the List of Icon and click PHP Settings
5. Scroll down and type the php directives
6. Once done, click OK

Here is the list of php.ini directives : http://php.net/manual/en/ini.list.php