The pcntl_fork() function
creates a child process that differs from the parent process
only in it's PID and PPID. Please see your system's fork(2)
man page for specific details as to how fork works on your
system.
On success, the PID of the child process is returned in
the parent's thread of execution, and a 0 is returned in the
child's thread of execution. On failure, a -1 will be
returned in the parent's context, no child process will be
created, and a PHP error is raised.
See also pcntl_waitpid() and
pcntl_signal().