I have a PHP script that resizes .jpg, .gif, and .png files to a bounding box.

            $max_width = 500;
            $max_height = 600;
            $filetype = $_FILES["file"]["type"];
            $source_pic = "img/" . $idnum;
            if($filetype == "image/jpeg")
            {
                $src = imagecreatefromjpeg($source_pic);                    
            } else if($filetype == "image/png")
            {
                $src = imagecreatefrompng($source_pic);                 
            } else if($filetype == "image/gif")
            {
                $src = imagecreatefromgif($source_pic);
            }
            list($width,$height)=getimagesize($source_pic);
            $x_ratio = $max_width / $width;
            $y_ratio = $max_height / $height;

            if( ($width <= $max_width) && ($height <= $max_height) )
            {
                $tn_width = $width;
                $tn_height = $height;
            } else if (($x_ratio * $height) < $max_height)
            {
                $tn_height = ceil($x_ratio * $height);
                $tn_width = $max_width;
            } else {
                $tn_width = ceil($y_ratio * $width);
                $tn_height = $max_height;
            }

            $tmp = imagecreatetruecolor($tn_width,$tn_height);
            imagecopyresampled($tmp,$src,0,0,0,0,$tn_width, $tn_height,$width,$height);
            $destination_pic = "img/thumbs/" . $idnum . "thumb";
            if($filetype == "image/jpeg")
            {
                imagejpeg($tmp,$destination_pic,80);
            } else if($filetype == "image/png")
            {
                imagepng($tmp,$destination_pic,80);
            } else if($filetype == "image/gif")
            {
                imagegif($tmp,$destination_pic,80);
            }
            imagedestroy($src);
            imagedestroy($tmp);

The script works fine with jpeg and gif but when running on a png the file will be corrupted.

Is there anything special I need to use when working with a png? I have never worked with this sort of thing in PHP so I'm not very familiar with it.