PHP调整大小图像功能创建漂亮的jpg文件，但可怕的png文件

Question

以下PHP功能适用于jpg文件。然而，它会创建像素化和太裁剪的png文件。

如何更改功能以创建质量更高的png文件？

/**
 * Resize image (while keeping aspect ratio and cropping it off sexy)
 *
 *
 * @param string $source_image The location to the original raw image.
 * @param string $destination The location to save the new image.
 * @param int $final_width The desired width of the new image
 * @param int $final_height The desired height of the new image.
 * @param int $quality The quality of the JPG to produce 1 - 100
 *
 *
 * @return bool success state
 */
public static function resizeImage($source_image, $file_ext, $destination, $final_width = 470, $final_height = 470, $quality = 85)
{
    $image = imagecreatefromstring(file_get_contents($source_image));

    $width = imagesx($image);
    $height = imagesy($image);

    if (!$width || !$height) {
        return false;
    }

    $original_aspect = $width / $height;
    $final_aspect = $final_width / $final_height;

    if ($original_aspect >= $final_aspect) {
       // horizontal image
       $new_height = $final_height;
       $new_width = $width / ($height / $final_height);
    } else {
       // vertical image
       $new_width = $final_width;
       $new_height = $height / ($width / $final_width);
    }

    $thumb = imagecreatetruecolor($final_width, $final_height);

    // Resize and crop
    imagecopyresampled($thumb,
                       $image,
                       0 - ($new_width - $final_width) / 2, // Center the image horizontally
                       0 - ($new_height - $final_height) / 2, // Center the image vertically
                       0, 0,
                       $new_width, $new_height,
                       $width, $height);

    // if the file is a jpg file
    if ($file_ext == 'jpg') {
        // add '.jpg' to file path, save it as a .jpg file with our $destination_filename parameter
        $destination .= '.jpg';

        imagejpeg($thumb, $destination, $quality);
    // if the file is a png file
    } elseif ($file_ext == 'png') {
        // add '.png' to file path, save it as a .jpg file with our $destination_filename parameter
        $destination .= '.png';

        imagepng($thumb, $destination);
    }

    if (file_exists($destination)) {
        return true;
    }
    // default return
    return false;
}

我对图像文件的知识非常有限，因此我不明白为什么这个函数适用于jpg-而不适用于png文件。

我会非常感谢任何帮助！