Mikko Koppanen has a quick post today showing how to use PHP and Imagick to create an image of text filled with another image layer behind it:

The fill pattern is used to annotate text but the named pattern could also be used to fill any shapes that allow fill to be specified (include circles, ellipses, rectangles, polygons etc etc).

His example is pretty simple (with a "Hello World!" output) that pulls in the background image, creates the composite layer over it, adds the text to it and sets the fill to the background image's layer. The whole this is wrapped up and output as a PNG file.