1. Login to Plesk Panel.
2. Select "Domains" under "Hosing services".
3. Select the domain.
4. Click "Websites and Domains" tab.
5. Click the domain / Click the option "Hosting Settings".
6. Select "Fast CGI application" from the drop down menu in the option 'PHP support (under 'Web Scripting and Statistics').
7 Click on 'OK' button.