[desmobob]

Sounds like a PHP resource leak.
A temporary fix would be sessions configured to time-out sooner rather than trying to recycle existing sessions open that aren't in use. We've seen a lot of code using JRUN on Win32 and Python (PHP) - on Freebsd and Redhat that have been waiting too long for the Ack back from a client (that never turns up) - resulting in time-out ..... waiting and ... waiting until ... bam ! No more resources availbable for more sessions available.