PHP script for adding watermarks to animated GIFs

We currently have a PHP script that adds watermarks to JPEG files, but we need one to add those watermarks to animated GIF files as well. The script we have now adds a text in a rectangle to the image (width = 100%) and requires some input parameters: the text, the number of pixels from the bottom, the height of the rectangle, the margin between the text and the rectangle borders, the colors of the text, border and rectangle background and the opacity of the watermark. The new script should work in the same way. It should work with GIFs up to 100MB in size and the new size of the animation, although it will be obviously bigger, needs to be as small as possible. We have limited access to add PHP libraries and complex scripts to our application/server, but basic plugins should be available. We work with PHP 5.6. If you need further information, do not hesitate to contact me.