$url="/images/convert.gif";  //url of image you want to convert (you will have to make an image upload script if you want to get these from user's harddrives)
$from="gif";  //convert from:  gif | png | jpeg
$to="png";  //convert to:  gif | png | jpeg
//------------------------------------------------------
header("Content-Type: image/".$to);
@{"image".$to}({"imagecreatefrom".$from}($url));