eAccelerator

From what I can tell eAccelerator is the best free php accelerator.

Here are my notes on how to set the control variables (in php.ini):

;linux 2.6 has 32meg limit. If you have the memory and lots of modules try to set even higher e.g to 64
eaccelerator.shm_size="32"

; people have suggested that putting this directory on a fast separate disk will help
; you shouldn't be generating that much activity on this disk anyway because of the shared memory configured above
eaccelerator.cache_dir="/Applications/MAMP/tmp/eaccelerator"
eaccelerator.enable="1"
eaccelerator.optimizer="1"

; You can set this to 0 for production sites. 1 means checks modification times so you don't need to restart web server
; if you are editing your php. You aren't editing php scripts on live sites are you?
eaccelerator.check_mtime="1"

eaccelerator.debug="0"
eaccelerator.filter=""
eaccelerator.shm_max="0"
eaccelerator.shm_ttl="3600"
eaccelerator.shm_prune_period="0"

; set this to 1 if you want to just use the memory cache: good if you have configured a large one
eaccelerator.shm_only="0"

eaccelerator.compress="1"
eaccelerator.compress_level="9"