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);
}