Re-read the OP - the problem is with threads. Switch apache to a prefork mpm - is a good solution, but replace apache with simple and more efficient setup is a

Re-read the OP - the problem is with threads. Switch apache to a prefork mpm - is a good solution, but replace apache with simple and more efficient setup is a much better solution. nginx + fpm isn't a silver bullet, but it is much more efficient an flexible solution. And avoiding using threads with such a terrible mess as PHP is also good solution, which will save you a lot of time and effort. Just think about how many things will be dynamically loaded, and how much of this code is really thread-safe and properly tested. btw, I bet there is a mere sigsegv.