PHP应用程序性能

First I want to say that I'm using Drupal as CMS and I know that there is separate Drupal stackexchange site. But my problem is not Drupal specific, it's not in User or Advanced User level. It's PHP and Server related. OK now problem.

I have developed website which is not launched yet. Am getting out of memory errors random times. And sometimes server gets crashed. Helps rebooting. There is no other people using App so no heavy load. Particulary am exceeding privmmpages limit. I have tried some general things - increasing/decreasing PHP memory limit, looking in error logs, logging slow MySQL queries. Nothing... Same.

I have ran 'top' linux command. There is 4-5 apache processes depending on browser requests. Which MEM usage(%) are 10, 5, 4, 3, 0.5. two processes are running >10hr. After restarting apache I got +40% free memory.

Here some questions and mysts for me.

Why that two processes are running so long when there is no active request from browser? And how can I prevent them?
Why I got +40% free memory after restarting when I had 10+5+4+3+0.5 memory used by apache? This should not be equal?
Can this be a memory leak? How can I detect them?
What techniques I should use to step down from higher levels to low levels? Imagine I have 'memory leak' in one of my function, how should I get him in whole application?
How can I benchmark my particulary functions for memory and CPU usage?
Why server is crashing? Even basic httpd restart is returning "fork: Cannot allocate memory". Can this be a symptom of memory leak?

Please answer point by point.

Sounds like you may have an infinite loop somewhere or your not releasing resources when dealing with things such as GD.

Linux keeps things in RAM while there is free ram, if there is a sudden need from another process for RAM, and the ram is not being held in use, Linux will free/swap it for the application in need. Check the output of "free" and you will notice a cached column that indicates how much is just cached and can be released at any time.