code: public static function isTransparentPng($fileName)
      {
          $bytes = file_get_contents($fileName, false, null, 24, 2);    // read 24th and 25th bytes
          $byte24 = ord($bytes{0});
          $byte25 = ord($bytes{1});
          if ($byte24 === 16 || $byte25 === 6 || $byte25 === 4) {
              return true;
          } else {
              $content = file_get_contents($fileName);
              return strpos($content, 'tRNS') !== false;
          }
      }
docs: Returns true if the given PNG file contains transparency information.
  
  @param string $fileName
  @return bool