I needed to run a PHP script in the background on my Ubuntu 8.04 server, but didn’t know how to do this. Googling around I found the following script to daemonize PHP scripts on bipinb.com:

<?php
    declare(ticks=1);
    $pid = pcntl_fork();

    if ($pid == -1) {
        die("could not fork");
    } else
    if ($pid) {
        exit();
        // we are the parent
    } else {
        // we are the child
    }

    // detatch from the controlling terminal

    if (posix_setsid() == -1) {
        die("could not detach from terminal");
    }

    $posid=posix_getpid();
    $fp = fopen("/var/run/process.pid", "w");
    fwrite($fp, $posid);
    fclose($fp);
    // setup signal handlers
    pcntl_signal(SIGTERM, "sig_handler");
    pcntl_signal(SIGHUP, "sig_handler");
    // loop forever performing tasks
	// **** place your main PHP code here ****
    while (1) {
        // **** place the PHP code that loops in here ****
    }

    fclose($fp);

    function sig_handler($signo) {
        switch ($signo) {
            case SIGTERM:
                // handle shutdown tasks
                exit;
                break;
            case SIGHUP:
                // handle restart tasks
                break;
            default:
                // handle all other signals
            }

    }
?>

I inserted my code and ran the script from the console. It all looked good, but found that whenever I left the console, the script would stop.
After some more googling I ended up on kevin.vanzonneveld.net, where I found a lot of information on creating daemons in PHP. In the troubleshooting area I found the following:

Don’t use echo()
Echo writes to the STDOUT of your current session. If you logout, that will cause fatal errors and the daemon to die. Use System_Daemon::log() instead.

This was the problem… in my PHP code I had some echo statements. When I deleted them, my PHP daemon was (and stayed) alive!