How to edit PHP settings for domain?
With Plesk 10.4 and newer versions, it can be done from the 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
In order to change PHP version, which is used for domain, check the #213909265 article.