在PHP中使用GD，如何在PNG和GIF文件上制作透明的PNG水印? (JPG文件工作正常)

我有一个图像(我们称它为原始图像)，在上面我要给另一个图像加水印(我们称它为 logo ).
徽标是透明的PNG，而原始图片可以是png，jpg或gif.
我有以下代码:

I have an image (let's call it original image) on which I want to watermark another image (let's call it logo).
The logo is a transparent PNG, whereas the original image can be png, jpg, or gif.
I have the following code:

function watermarkImage($originalFileContents, $originalWidth, $originalHeight) {
    $logoImage = imagecreatefrompng('logo.png');
    imagealphablending($logoImage, true);

    $logoWidth  = imagesx($logoImage);  
    $logoHeight = imagesy($logoImage);

    $originalImage = imagecreatefromstring($originalFileContents);

    $destX = $originalWidth  - $logoWidth;
    $destY = $originalHeight - $logoHeight;

    imagecopy(
        // source
        $originalImage,
        // destination
        $logoImage,
        // destination x and y
        $destX, $destY,
        // source x and y
        0, 0,
        // width and height of the area of the source to copy
        $logoWidth, $logoHeight
    );
    imagepng($originalImage);
}

仅当原始图像是JPG文件时，此代码才能很好地工作(良好=保持徽标的透明度).
当原始文件是GIF或PNG时，徽标具有纯白色背景，这意味着透明度不起作用.

This code works good (good = keep the transparency of the logo) only when the original image is a JPG file.
When the original file is a GIF or PNG, the logo has a solid white background, meaning the transparency is not working.

为什么?我需要更改什么才能生效?
谢谢

Why ? What do I need to change so it'll work ?
Thanks

更新:
这是我的重新编码版本:

UPDATE:
Here is my recoded version:

function generate_watermarked_image($originalFileContents, $originalWidth, $originalHeight, $paddingFromBottomRight = 0) {
    $watermarkFileLocation = 'watermark.png';
    $watermarkImage = imagecreatefrompng($watermarkFileLocation);
    $watermarkWidth = imagesx($watermarkImage);  
    $watermarkHeight = imagesy($watermarkImage);

    $originalImage = imagecreatefromstring($originalFileContents);

    $destX = $originalWidth - $watermarkWidth - $paddingFromBottomRight;  
    $destY = $originalHeight - $watermarkHeight - $paddingFromBottomRight;

    // creating a cut resource
    $cut = imagecreatetruecolor($watermarkWidth, $watermarkHeight);

    // copying that section of the background to the cut
    imagecopy($cut, $originalImage, 0, 0, $destX, $destY, $watermarkWidth, $watermarkHeight);

    // placing the watermark now
    imagecopy($cut, $watermarkImage, 0, 0, 0, 0, $watermarkWidth, $watermarkHeight);

    // merging both of the images
    imagecopymerge($originalImage, $cut, $destX, $destY, 0, 0, $watermarkWidth, $watermarkHeight, 100);
}