Startinga shell process with a life of its own

Which PHP method allows me to start a shell process from a web page script and let it proceed to its conclusion even though I end the page. Most of the various execute functions seem to wait for the process to finish before PHP continues. I don't understand the ones with open/close functions. Will opening a process and failing to close it let it run after the page ends? Will failing to close cause other problems? Can you point me to example code? The shell commands I want to run are typically themselves PHP programs whose execution time is far too long to complete in a browser context. John Gunther Bucks vs Bytes Inc