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.

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 continues
echo "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.

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.