How to edit PHP settings for a domain?
From Plesk 10.4 and newer versions, it can be done from web interface:
Open desired domain in Plesk.
Go to Websites & Domains tab.
Click on the domain name in the list of domains.
Go to the PHP Settings tab:
Or for newer versions:
This way, a unique PHP configuration can be created for this website (apart from the other sites of the subscription).
Learn more about custom PHP configuration in Plesk 12.0: Custom PHP Configuration