Recently I've noticed that httpd processes started to consume massive amounts of memory - after some time pretty much using almost all of the 2GB of RAM the server has and I don't have any memory left for other stuff. Here's what top tells me:

26409 apache 15 0 276m 152m 28m S 0 7.4 0:59.12 httpd
26408 apache 15 0 278m 151m 28m S 0 7.4 1:03.80 httpd
26410 apache 15 0 277m 149m 26m S 0 7.3 0:57.22 httpd
26405 apache 15 0 276m 148m 25m S 0 7.3 0:59.20 httpd
26411 apache 16 0 276m 146m 23m S 0 7.2 1:09.18 httpd
17549 apache 15 0 276m 144m 23m S 0 7.0 0:36.34 httpd
22095 apache 15 0 276m 136m 14m S 0 6.6 0:30.56 httpd

It seems to me that each httpd process does not free the memory after handling a request. So they all sit at ~270MB which is BAD. Is there a way for me to know where all the memory goes and why it stays that way? I haven't done any server tweaking lately, so I'm sure it's not me who messed something up (haven't had the problem before).

The server is used to serve PHP apps.

EDIT: Apache is configured with prefork module and MaxRequestsPerChild is set to 4000.