I... think this works. :D
Note that the final & will be ignored, so the exec will block. You can test it with php "-rexec('sleep 5 >nul 2>&1 &');"
There doesn't seem to be a way to spawn in background from php without an external helper program.
pcntl_fork(); That won't work on windows, but on the other hand & will neither.
I don't think you will find many hosts with pcntl extension enabled.
Hmm, it does work on Ubuntu for both Michael and me. What distro are you using?