Mengapakah imej PNG menjadi hitam dengan `imagecreatefrompng()` dalam PHP?

<code class="php">function cropImage($nw, $nh, $source, $stype, $dest) {
  // ...
  switch($stype) {
    case 'png':
      $simg = imagecreatefrompng($source);
      break;
    // ...
  }
  // ...
}</code>

<code class="php">// Before imagecreatetruecolor()

$dimg = imagecreatetruecolor($width_new, $height_new); // png ?: gif

// Additional steps for PNG and GIF

switch ($stype) {

    case 'gif':
    case 'png':
        // Black color
        $background = imagecolorallocate($dimg , 0, 0, 0);
        // Remove black from placeholder
        imagecolortransparent($dimg, $background);
        // Turn off alpha blending
        imagealphablending($dimg, false);
        // Turn on alpha channel saving
        imagesavealpha($dimg, true);
        break;

    default:
        break;
}</code>