I recently installed WHMCS on a VPS running Plesk and had some issues configuring Cron.
the suggested cron command was
*/5 * * * * php -q /var/www/vhosts/MYDOMAIN/whmcs/crons/cron.php
However this invokes the CLI version of php installed on the server and failed with the complaint that the ioncube loader version was incorrect and could not decode the cron file.
The domain where WHMCS is installed is running php 7 but the cli php on the server itself is stuck at version 5.3 and upgrade not available in the yum repositories for centos6.
Altering the cron command to point at the correct php binary in use of the domain resolved the issue
/opt/plesk/php/7.0/bin/php -q /var/www/vhosts/MYDOMAIN/whmcs/crons/cron.php
You also need to set the /path/to/WHMCS/ in the config in the crons directory
Hope this helps other Plesk users to get their configuration working