pcntl_exec

The pcntl_exec() function works exactly like the standard (unix-style) exec() function.  It differs from the regular PHP exec() function in that the process calling the pcntl_exec() is replaced with the process that gets called.  This is the ideal method for creating children.  In a simple example (that does no error checking):switch (pcntl_fork()) {  case 0:    $cmd = "/path/to/command";    $args = array("arg1", "arg2");    pcntl_exec($cmd, $args);    // the child will only reach this point on exec failure,    // because execution shifts to the pcntl_exec()ed command    exit(0);  default:    break;}// parent continuesecho "I am the parent";--since this is not being executed through a shell, you must provide the exact path from the filesystem root.  Look at the execve() man page for more information.

Some people might find it useful to run other program using the same process as a different user. This is very usefull if the script is running under root. Here is a simple code to achieve that under *nix PHP CLI:#!/usr/bin/php -q<?php//Enter run-as user below (argument needed to be passed when the script is called), otherwise it will run as the caller user process.$username = $_SERVER['argv'][1];$user = posix_getpwnam($username);posix_setuid($user['uid']);posix_setgid($user['gid']);pcntl_exec('/path/to/cmd');?>I use this as a part of socket program so that a program can be run under different user from remote location.

As a side note, if I'm reading the comments below correctly, you should not run this if you're using a PHP webserver module, as it will replace the webserver's process with whatever process you're telling it to run.