Change gif image with php

Hi list I have some buttons made in photoshop in .gif format. This buttons have round corner, feather, multiple colors etc. There's a way to open it with php and write some text to it. I know that is possible to create images, but what about to change them??? I need to do this because the text that will fill the buttons come from mysql and change everyday. Thank's in advance Rodrigo ps: If someone want to see the button I'm talking look in http://www.celebnet.com.br/home.php under the "AS ++".