Log into Plesk (click here), and then select the appropriate website from the list, then select Dashboard and PHP.
To change PHP version:
On that page, you can then change PHP versions using the dropdown menu. Ensure you click Apply or OK at the bottom of the page after making any changes.
To change PHP features:
You can also change a variety of settings such as the memory limit, maximum execution time, maximum file upload sizes, include_path, and error logging settings from this page. As above, ensure you click Apply or OK at the bottom of the page after making any changes.
