How can one use multi threading in PHP applications

Is there a realistic way of implementing a multi-threaded model in PHP whether truly, or just simulating it. Some time back it was suggested that you could force the operating system to load another instance of the PHP executable and handle other simultaneous processes. The problem with this is that when the PHP code finished executing the PHP instance remains in memory because there is no way to kill it from within PHP. So if you are simulating several threads you can imagine whats going to happen. So I am still looking for a way multi-threading can be done or simulated effectively from within PHP. Any ideas?