This was written by Russell Beattie; I copied it here from a pastebin to give it a permanent home.

unicorn.php

<?
/*
Simple preforking echo server in PHP.
Russell Beattie (russellbeattie.com)

PHP port of http://tomayko.com/writings/unicorn-is-unix
*/

/* Allow the script to hang around waiting for connections. */
set_time_limit(0);

# Create a socket, bind it to localhost:4242, and start
# listening. Runs once in the parent; all forked children
# inherit the socket's file descriptor.
$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
socket_bind($socket,'localhost', 4242);
socket_listen($socket, 10);

pcntl_signal(SIGTERM, 'shutdown');
pcntl_signal(SIGINT, 'shutdown');

function shutdown($signal){
    global $socket;
    socket_close($socket);
    exit();
}


# Fork you some child processes. In the parent, the call to
# fork returns immediately with the pid of the child process;
# fork never returns in the child because we exit at the end
# of the block.
for($x = 1; $x <= 3; $x++){
    
    $pid = pcntl_fork();
    
    # pcntl_fork() returns 0 in the child process and the child's
    # process id in the parent. So if $pid == 0 then we're in
    # the child process.
    if($pid == 0){

        $childpid = posix_getpid(); 
        
        echo "Child $childpid listening on localhost:4242 \n";

        while(true){
            # This is where the magic happens. accept(2)
            # blocks until a new connection is ready to be
            # dequeued.
            $conn = socket_accept($socket);

            $message = socket_read($conn,1000,PHP_NORMAL_READ);
            
            socket_write($conn, "Child $childpid echo> $message");
        
            socket_close($conn);
        
            echo "Child $childpid echo'd: $message \n";
        
        }

    }
}


#
# Trap interrupts, write a note, and exit immediately in
# parent. This trap is not inherited by the forks because it
# runs after forking has commenced.
try{

    pcntl_waitpid(-1, $status);

} catch (Exception $e) {

    echo "bailing \n";
    exit();

}