MySQL Performance Blog - Wed, 21/05/2008 - 5:55am
Sometimes due to programming error or due to very complex query you can get your PHP script running too long, well after user stopped waiting for the page to render and went browsing other sites.
Looking at Server-Status I've seen scripts executing for hours sometimes which is obviously the problem - they take Apache Slot, MySQL Connection and other resources.
I had discussion today who was thinking Apache would kill the script after "Timeout" specified in Apache configuration is reached - this was not my experience so I decided to run couple of tests to check it.