Newbie Question: PHP/Appache/WINXP/MySQl

Does the PHP script have to be already running to communicate send a HTTP Post varibale to it (from a DHTML client browser)? Or can it initiate the HTTP Post fron the client that activates the PHP script? I know this must seems simple but I'm having troble receiving these varible from my client. When I find out the value of the vaiable I stick it into a MySQL table. Both client DHTML and server PHP work well alone but I can't seem to et the connection going using localhost.