In order to alter the PHP version per website, navigate to the User Level, select the domain name, click "Domain Setup" under "Your Account", click the domain name and scroll down. As demonstrated in the screenshot below, you will now be able to choose a PHP version. Note that after changing the version, it can take up to 1 minute to process the changes. A scheduled restart of the web server is added to the DirectAdmin task queue.
If you are not able to see the multiple versions of PHP please raise a support ticket and let us know that you do want to have multiple PHP versions installed.