PHP dynamic image creation

We are a promotional product company that prints a wide range of products with custom designs. We are developing an online design tool (javascript/html/php) and need a function/script (in PHP using GD libs) to create a print ready image file. The image width will be variable depending on design content but the required height will be given as part of the data sent to the script. There will be a mix of images and/or text. Data will be passed in using xml. Output must match *exactly* the design shown on screen. Files will be saved in a path/file given as part of the data sent to the script. Please feel free to ask any questions, however I think this is quite a straightforward project for someone very familiar with GD.