I assume this is your client. Offer them upgrade paths to something better than OpenCart or at least up to date version. If they refuse they are usually either a cheapskate or don't have the money so you won't usually lose much by dropping them.
Spend no time getting something insecure to work. Running PHP 5.6 simply isn't secure even if its in ionCube. Don't expose yourself to the security liability. If they don't like it they can go to some garbage host that still offers 5.6
Even if they are isolated from other clients it will still impact your service when a PHP 5.6 vuln lets a bot launch attacks from your server IP...