How does memory limit work?

Hi Everybody, I am trying to prevent my Linux machine from crashing by the use of memory_limit, but is does not seem to work as I expect it to: When a php-script includes itself recursively by accident it grabbes all memory available in a matter of seconds, although I set max_execution_time=30 and memory_limit=8388608. After 30 seconds of execution the apache terminates the request, but the script keeps on running and allocating memory (far more than 8 MB). Do I misunderstand the memory_limit option? Is there an other way to limit memory usage besides system wide memory quotas? Thanks Ralf