it not work with png created a thumb png but haven't data , like null data :D with jpg , jpeg still working without error why ?

 function thumbnail($pathtoFile,$thumWidth,$pathtoThumb)
        {

             //infor of image
             $infor = pathinfo($pathtoFile);

             // Setting the resize parameters 
             list($width, $height) = getimagesize($pathtoFile); 

             $modwidth = $thumWidth; 
             $modheight = floor( $height * ( $modwidth / $width )); 


             // Resizing the Image 
             $thumb = imagecreatetruecolor($modwidth, $modheight);

             switch(strtolower($infor['extension']))
             {
                case 'jpeg':
                case 'jpg':
                     $image = imagecreatefromjpeg($pathtoFile); 
                break;

                case 'gif':
                     $image = imagecreatefromgif($pathtoFile); 
                break;

                case 'png':

                    $image = imagecreatefrompng($pathtoFile); 
                break;
             }

             imagecopyresampled($thumb, $image, 0, 0, 0, 0, $modwidth, $modheight, $width, $height); 


             switch(strtolower($infor['extension']))
             {
                case 'jpeg':
                case 'jpg':
                     imagejpeg($thumb,$pathtoThumb, 70); 
                break;

                case 'gif':
                     imagegif($thumb,$pathtoThumb, 70); 
                break;

                case 'png':

                    imagepng($thumb,$pathtoThumb, 70); 
                break;
             }

             //destroy tmp
             imagedestroy($thumb);
        }