Check the PCNTL library. It may help you to emulate some thread behavior.
Also there's this class:
"This class can emulate the execution
of program threads using separate HTTP
requests to the same script.
It establishes an HTTP connection to
the same Web server to execute the
same PHP script. It sends a request
passing the name a function to execute
and an argument to be passed to that
function.
The requested script executes some
code that detects the thread execution
request and calls the specified
function.
When the thread request script ends,
the return values of the called
function is returned as a serialized
string.
The calling script can execute other
tasks while the thread script runs.
The results may be collected later
when the thread script ends."
