Adding PNG Watermarks to JPEGs with PHP

This took up a good chunk of my afternoon. I have a client who wants to upload JPEG images to his website and have them automatically resized and watermarked with copyright information to discourage image theft. Resizing is a piece of cake --- I've done that before --- but getting the alpha transparency of a 24-bit PNG to overlay correctly was maddening.

It turns out that the solution is actually really simple. The code is below:

 1$photo = imagecreatefromjpeg('original.jpg');
 2$watermark = imagecreatefrompng('watermark.png');
 3 
 4// This is the key - without ImageAlphaBlending on, the PNG won't render correctly. 
 5imagealphablending($photo, true);
 6 
 7// Copy the watermark onto the master, $offset px from the bottom right corner.
 8$offset = 10;
 9imagecopy(
10    $photo,
11    $watermark,
12    imagesx($photo) - imagesx($watermark) - $offset,
13    imagesy($photo) - imagesy($watermark) - $offset,
14    0,
15    0,
16    imagesx($watermark),
17    imagesy($watermark)
18);
19 
20// Output to the browser - note that on a production website, you should save the image to the filesystem and serve it directly.
21header('Content-Type: image/jpeg');
22imagejpeg($photo);

Note: It's a bad idea to do this on the fly for each visitor. I strongly recommend watermarking an image once and then using that image for your website.