How to interrupt function execution after X seconds?

I execute a system call via proc_open() that usually completes in less than one minute Sometimes, it hangs for many hours I want to exit this parent script after thirty (30) minutes with the error description on STDERR and I also want that system call process killed at this point Although, a test script using ticks and pcntl_alarm(1800) works using a while() loop, this does NOT work with my production system call What am I missing? Please, advise. Thank you ~ Mike