php：在屏幕输出之前运行的shellscript

I'm currently putting together a small web-based GUI to generate kickstart-scripts. I got a confirmation page that's sending the relevant data via POST to the PHP-page where the actual shell script is called to build the iso. So far it's working, but the page seems to execute the script before it outputs anything else (for example, the 'echo' I put in at the beginning of the page ...), and I'm absolutely clueless why. Would anyone care to enlighten me?

Here's the code to the PHP-page that's executing the shell script ...

echo 'Generating your ISO; this might take a while...';
sleep(20);

if (!isset($_POST['auth'])) {
        $ad = 'N';
        }
else {
        $ad = 'Y';
}

if (!isset($_POST['oracle'])) {
        $oracle = 'N';
        }
else {
        $oracle = 'Y';
        }

if ((!isset($_POST['ip'])) or (!isset($_POST['hostname'])) or (!isset($_POST['rhsel'])) or (!isset($_POST['submit'])) or (!isset($_POST['gw'])) or (!isset($_POST['nm']))) {
        die('Please use the correct form !');
}

if (isset($_POST['ip'])) {
        $ip = trim($_POST['ip']);
        }

if (isset($_POST['gw'])) {
        $gw = trim($_POST['gw']);
        }

if (isset($_POST['nm'])) {
        $nm = trim($_POST['nm']);
        }

if (isset($_POST['hostname'])) {
        $hostname = trim($_POST['hostname']);
        }

if (isset($_POST['rhsel'])) {
        $rhsel = $_POST['rhsel'];
        }
passthru("/usr/bin/sudo /data/skripte/webconfig.sh $rhsel $oracle $ad $ip $gw $nm $hostname 2>&1");

PHP scripts accessed via a browser are request-response, meaning all processing is done on the server prior to headers and content being sent to the client. This means you will not get a continually updating output like you would see on the command line. There is no way around this. Sorry.