Como decia BVis .. :
Mírate "Process Control Functions".
http://www.php.net/manual/en/ref.pcntl.php
Yo no estoy muy puesto en el tema .. pero por ahi se ven funciones como:
pcntl_exec - Executes specified program in current process space
pcntl_fork - Forks the currently running process
pcntl_signal - Installs a signal handler
pcntl_waitpid - Waits on or returns the status of a forked child
pcntl_wexitstatus - Returns the return code of a terminated child
pcntl_wifexited - Returns TRUE if status code represents a successful exit
pcntl_wifsignaled - Returns TRUE if status code represents a termination due to a signal
pcntl_wifstopped - Returns TRUE if child process is currently stopped
pcntl_wstopsig - Returns the signal which caused the child to stop
pcntl_wtermsig - Returns the signal which caused the child to terminate
(previo compilacion PHP con: --enable-pcntl .. No funciona en Windows.)
Un saludo,