Simple question (what's ISAPI ?)

Hi folks. Can someone tell me what ISAPI is? I've requested how I could run PHP faster, if 20 USER's access my files at the same time. In my case, PHP run's the php.exe (ca. 5 MB) every time seperatly, and this eats my win-resources!!!!! Someone guides me to use it as an ISAPI module.... but WHAT is that? and HOW can I do that under my win-operation system. Thank's for any help! Tribun (Patrick) --- mp3o.net