alpha with the gd library

Hello all.
I'm developing a Flash app in which the user traces his or her face. Done. The coordinates are being sent to a PHP page which fills a polygon. Done. Now what I would like to do is instead of filling the polygon, convert it to alpha. Can this be done? I'm attaching my code and a screenshot of what I've done so far.
Any help is appreciated.

Code:

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<?
    /* INITIALIZE A VARIABLE */
        $coordinates=substr($_GET['c'],0,strlen($_GET['c'])-1);
    /* BUILD AN ARRAY */
        $temp=split(',',$coordinates);
    /* CREATE THE IMAGE */
        $image=imagecreatetruecolor(400, 370);
    /* USE A FILL */
        $fill_color=imagecolorallocate($image, 255, 255, 255);
    /* DRAW IT */
        imagefilledpolygon($image, $temp, (count($temp)/2), $fill_color);
    /* SET THE HEADER */
        header('Content-type: image/png');
    /* FLUSH THE IMAGE */
        imagepng($image);
        imagedestroy($image);
?>

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