Hi Antonio,
Im not sure about the php issue you are referencing, however if you use the (free) atomic packages and the atomic php-panda you can install the 5.4, 5.5, 5.6, and 7.0 packages of php and allow your users to chose which version of php they use. I think that Plesk in general also allows this behavior so you can do it from them as well if you prefer. That may resolve the issue with php for your customer and thus allow you to upgrade plesk to the latest version. A lot of bug fixes have been fixed (and the php switcher may even be part of 12.5).