PHP add iTXt comment to a PNG image

Question

I have looked around for this everywhere. I know it can be done using some libraries in .net, but I really want my script to generate a \"marked\" image. The reason is that we a

Answer 1

A suggestion: If you have a fixed iTXt chunk that you want to add to an image, a quick and dirty procedure could be to simply insert it just before the IEND chunk (12 bytes) of the original image. This should work, because the iTXt can be placed before of after the image data. Of course, this does not check if the chunk as already there.

Here's an example code, using the tEXt chunk (slightly simpler), it needs some polishing but it basically works:

<?php   

    addTextToPngFile("x.png","x2.png","Watermark","Hi this is a TEXT test");

    function addTextToPngFile($pngSrc,$pngTarget,$key,$text) {
        $chunk = phpTextChunk($key,$text);
        $png = file_get_contents($pngSrc);
        $png2 = addPngChunk($chunk,$png);
        file_put_contents($pngTarget,$png2);
    }

    // creates a tEXt chunk with given key and text (iso8859-1)
    // ToDo: check that key length is less than 79 and that neither includes null bytes
    function phpTextChunk($key,$text) {
        $chunktype = "tEXt";
        $chunkdata = $key . "\0" . $text;
        $crc = pack("N", crc32($chunktype . $chunkdata));
        $len = pack("N",strlen($chunkdata));
        return $len .  $chunktype  . $chunkdata . $crc;
    }

    // inserts chunk before IEND chunk (last 12 bytes)
    function addPngChunk($chunk,$png) {
        $len = strlen($png);
        return substr($png,0,$len-12) . $chunk . substr($png,$len-12,12);
    }

?>