Fatal error: Out of memory, but I do have plenty of memory (PHP)

Question

Since my question is getting longer and longer, I decide to re-write the whole question to make it better and shorter.

I run my website on dedicated server with 8GB

Answer 1

Install xdebug and enable profiler trigger. Generate a profiler file, then post the cachegrind file if you still would not be able to tell the source of the problem.

EDIT: profiler file of the page where the memory leak happens of course!