https://support.plesk.com/hc/en-us/articles/115000497853–How-to-add-a-custom-PHP-version-in-Plesk-for-Linux-

Sample command :

plesk bin php_handler –add -displayname 5.6.4 -path /usr/local/php564/bin/php-cgi -phpini /usr/local/lib/php.ini -type fastcgi -id 5.6.4-custom -clipath /usr/local/php564/bin/php