shm_size
Total memory available. Want this as large as possible.
ttl
set to 0 by default, which means once old files are held in cache, they stay forever
If your memory is getting full, or close to full, increase this number
Othewise, the higher the number the better (and disabling with 0 is best!)
num_files_hint
Set low by default (1000) - most frameworks have many more files
max_file_size
Set to 1M by default. If you want larger PHP files cached, turn this up.
stat
Faster, but means you have to manually clear APC cache when changing PHP files