Then I realized it was pretty amazing that it lasted that long in the first place.
this call came in late 2017 saying "hey, we migrated the site and it doesn't work". There was some change in PHP5.6 and the initial PHP4 code had some "by reference" class stuff that wasn't working any more. It was a small change to get it working again, but there was enough PHP4 in it that a move to PHP7 wasn't going to work without a lot of work. I'd suggested a rebuild, which... wasn't too well received. Pointing out that they'd had 16 years of functionality on a web codebase (with no other costs) was probably as much ROI/value as you'd ever be able to get out of anything.