The quick and dirty way is to create a semitransparent png that is your black bar, at whatever opacity, and use imagealphablending() to turn on blending and then lay it over the bottom of your image, then write your white text over that.
Of course the slightly better way is to create your own semitransparent black bar, by creating a new image, with alpha, and filling it with a custom created colour that is black with transparency, and using that.
but you'll basically want to be using: imagealphablending, imagecopy and imagesavealpha.
