Prefork
Every request gets its own (memory-separated) process
Worker
Multi-threaded Apache, uses threads instead of processes, is generally faster than prefork and might use less memory
In regards to PHP
As PHP is not thread-safe, the common suggestion is to install Apache with the “prefork” MPM.
Reference: http://www.php.net/manual/en/faq.installation.php#faq.installation.apache2
Note: Debian will automatically choose the right Apache version if you’re installing PHP.