[ Thanks to Lee
Schlesinger for this link. ]
“An Apache HTTP server can be configured to work in either of
two ways. Multi-Processing Module Prefork (MPM prefork) implements
a non-threaded, pre-forking web server that is appropriate for
sites that need to avoid threading for compatibility with
non-thread-safe libraries. Typically, any non-trivial Apache module
(such as mod_php, a module that Apache can load to serve PHP
dynamic pages) has some sort of non-thread-safe library or has
non-thread-safe code in it, so when in doubt use the MPM prefork.
In many distribution, including Debian and Red Hat, when you
install mod_php the system automatically installs the MPM prefork
version of the Apache package. MPM prefork is also the best MPM for
isolating each request, so that a problem with a single request
will not affect any other. A request generally corresponds to a
single process on a Linux server.”