FPM (FastCGI Process Manager) is an alternative PHP FastCGI implementation with some additional features (mostly) useful for heavy-loaded sites.

These features include:

advanced process management with graceful stop/start;

ability to start workers with different uid/gid/chroot/environment, listening on different ports and using different php.ini (replaces safe_mode);

stdout and stderr logging;

emergency restart in case of accidental opcode cache destruction;

accelerated upload support;

“slowlog” – logging scripts (not just their names, but their PHP backtraces too, using ptrace and similar things to read remote process’ execute_data) that are executed unusually slow;

fastcgi_finish_request() – special function to finish request and flush all data while continuing to do something time-consuming (video converting, stats processing etc.);

dynamic/static child spawning;

basic SAPI status info (similar to Apache mod_status);

php.ini-based config file.