I sometimes work on old PHP applications, some of which require versions as ancient as 5.4 and refuse to run on anything beyond that. They're not only difficult

I sometimes work on old PHP applications, some of which require versions as ancient as 5.4 and refuse to run on anything beyond that. They're not only difficult/impossible to install on modern systems, but also make it hard to onboard other people — you have to write a detailed setup document and then waste a couple of hours time for each new system. And then repeat that a few more years down the line. Why not write that document as a bunch of build instructions instead of free-form text, while also isolating the result from the rest of the system.