I have got to the bottom of the problem and it might help others, so here is the solution.
You cannot use either XCache or OpCache or any other caching system on a shared hosting system.
The one we were working with was suPHP, which as you may know works well, on a shared hosting system.
Unfortunately, due to the way shared hosting works, you cannot for security reasons use the accelerator caches, even if they are present in cPanel or equivalent.
You can switch the caches on, but they will slow down your code and provide a penalty not a benefit. Since they do not save any data, the cache is destroyed at the end of the invoked script.
Summary
On a shared system, with suPHP or equivalent, there is no possibility of using OpCache in PHP. Turn them off, they will only hurt performance.
