PHP实现对png图像进行缩放的方法(支持透明背景)

function smart_resize_image( $file, $width = 0, $height = 0, $proportional = false, $output = 'file', $delete_original = true, $use_linux_commands = false )
{
    if ( $height <= 0 && $width <= 0 ) {
      return false;
    }
    $info = getimagesize($file);
    $image = '';
    $final_width = 0;
    $final_height = 0;
    list($width_old, $height_old) = $info;
    if ($proportional) {
      if ($width == 0) $factor = $height/$height_old;
      elseif ($height == 0) $factor = $width/$width_old;
      else $factor = min ( $width / $width_old, $height / $height_old); 
      $final_width = round ($width_old * $factor);
      $final_height = round ($height_old * $factor);
    }
    else {    
      $final_width = ( $width <= 0 ) "Content-type: $mime");
        $output = NULL;
      break;
      case 'file':
        $output = $file;
      break;
      case 'return':
        return $image_resized;
      break;
      default:
      break;
    }
    switch ($info[2] ) {
      case IMAGETYPE_GIF:
        imagegif($image_resized, $output);
      break;
      case IMAGETYPE_JPEG:
        imagejpeg($image_resized, $output);
      break;
      case IMAGETYPE_PNG:
        imagepng($image_resized, $output);
      break;
      default:
        return false;
    }
    return true;
}