By the way, just to add, what is the benefits of prefork MPM over worker MPM. Say for a server with one core, and a server with 2 cores and a server with 4 cores? which one is better - prefork mpm or worker mpm?
All things work together for the good of those who love God - Romans 8:28
This implements a non-threaded, pre-forking web server that handles requests in a manner similar to Apache 1.3. It is appropriate for sites that need to avoid threading for compatibility with non-thread-safe libraries.
Basically Apache2 allows you to use different methods in how it manages the various incoming connections. One of the more efficient methods uses threads; but when running PHP you've been restricted to using the prefork MPM module since some extensions are not written in a way that will enable work in thread safe mode.
A better alternative will be to install Apache2 MPM ITK.