imagepng is not working, it creates a blank image and image does not compress

I am creating a thumbnail of myImage of various format (jpg, png). Here is my code

<?php
error_reporting(E_ALL);
//$file = 'H:/xampp/htdocs/myfolder/new.jpg';
$file = 'H:/xampp/htdocs/myfolder/phool.png';
$pathToSave = 'H:/xampp/myfolder/New/';
$sourceWidth =60;
$sourceHeight = 60;
$what = getimagesize($file);
$file_name = basename($file);
//print "MIME ".$what['mime'];
switch(strtolower($what['mime']))
{
    case 'image/png':
            $img = imagecreatefrompng($file);
            $img = imagecreatefromstring($img);
            $new = imagecreatetruecolor($what[0],$what[1]);
            imagecopy($new,$img,0,0,0,0,$what[0],$what[1]);
            header('Content-Type: image/png');
            imagepng($new,$pathToSave.$file_name);
            imagedestroy($new);
        break;
    case 'image/jpeg':
            $img = imagecreatefromjpeg($file);
            $new = imagecreatetruecolor($what[0],$what[1]);
            imagecopy($new,$img,0,0,0,0,$what[0],$what[1]);
            header('Content-Type: image/jpeg');   
            imagejpeg($new,$pathToSave.$file_name);
            imagedestroy($new);
        break;
    case 'image/gif':
        $img = imagecreatefromgif($file);
        break;
    default: die();
}

But, It is not working for PNG images.It saves a blank image in my destination $pathToSave = 'H:/xampp/htdocs/myfolder/New/'.

If I remove $img = imagecreatefromstring($img); then imagepng saves the image as it is but does not compress the image as imagejpg does.

$img = imagecreatefrompng($file); //Creates an image from a png file
$img = imagecreatefromstring($img); //Creates an image from a string

Basicly you are creating an image in memory using imagecreatefrompng($file) but then you overwrite the same variable using imagecreatefromstring($img) which doesn't take a GDImage variable as parameter so it returns a blank image (or null?).

Just remove imagecreatefromstring($img); and it will work.

$newImage = imagecreatetruecolor($newwidth, $newheight);
$source = imagecreatefrompng($fileurl);
imagecopyresized($newImage, $source, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
imagepng($newImage,$fileurl);
imagedestroy($newImage);
imagedestroy($source);