Creating a QR Code with a centered Logo in PHP with PHP QR Code Generator

I'm creating QR codes with PHP QR Code (http://phpqrcode.sourceforge.net/). It works well but now I need a free space for a custom graphic or logo in the center of it. And I want to do this without saving the image on the server. Has anyone a suggestion? What I've got so far is this:

<?php
     $param = $_GET['projectid']; 
     $divider = ",";

     $codeText = 'Projectname'.$divider.$param;

     // outputs image directly into browser, as PNG stream
     //QRcode::png($text, $outfile = false, $level = QR_ECLEVEL_L, $size = 3, $margin = 4, $saveandprint=false)
     QRcode::png($codeText, false, QR_ECLEVEL_H, 9, 2, true );
?>