The support guys have been fabulous at recommending various internal files that I can achieve my required customisations, such as fields.xml, or jscalendar lang files, etc, but I've noticed that these modifications get lost when I reinstall or upgrade PHPMaker, which is a problem for long-term maintainability. By the time the next PHPMaker upgrade comes out, I stand a very good chance of forgetting the tweaks I've made.
Please either add a list of such files to a "how to upgrade" document, or better yet, provide a central location for such overrides. I like this - it replicates the folder structure of the entire framework (as empty folders), so you just place a copy of any file you're replacing or adding in the relevant empty folder, and it loads that instead. Then you just zip up the whole overrides folder with your app, and you have a backup of all your code, and nothing gets lost.