If the error isn’t repairable by the user, blocking them from using the app entirely is mean. If the error screen has a message telling the user where to go to set their name, that’s fine but annoying. If the error screen tells the user they can’t use the app until someone checks a dashboard and sees a large enough increase in errors to investigate, that’s a bigger problem.
This reads like a dogmatic view of someone who hasn’t worked on a project that’s a million plus lines of code where something is always going wrong, and crashing the entire program when that’s the case is simply unacceptable.
tldr: undefined constants were treated as a string (+ a warning), so `$x = FOO` was `$x = "FOO"` + a warning if `FOO` was not a defined constant. Thankfully this feature was removed in PHP 8.