help needed with zipping a folder and downloading

This pops up a dialog box: and says and allows me to save the file but; it does not save it as a zip file. I also notice that it does not put the extension .zip and if i manually change the downloaded file name to a .zip and try to open it says the file is corrupt.

main page form
<form id="form2" name="form2" method="post" action="zip3.php">
      <label>Enter Username to get images for that user<br />
      <input type="text" name="userName" id="userName" />
      </label>
      <input name="Submit2" type="submit" />
    </form>
    <?php
$userName = $_POST['userName'];
$_SESSION['userName'] = $userName;
?>


zip3.php
<?php   
    $filename = "upload_test/".$_POST['userName'];
    $buffer = file_get_contents($filename);

    /* Force download dialog... */
    header("Content-Type: application/force-download");
    header("Content-Type: application/octet-stream");
    header("Content-Type: application/download");

    /* Don't allow caching... */
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");

    /* Set data type, size and filename */
    header("Content-Type: application/octet-stream");
    header("Content-Transfer-Encoding: binary");
    header("Content-Length: " . strlen($buffer));
    header("Content-Disposition: attachment; filename=$filename");

    /* Send our file... */
    echo $buffer;
?>