Quirks mode would create a lot more work for the maintainers that could be better used for advancing the language. If you really have an old codebase that canno

Quirks mode would create a lot more work for the maintainers that could be better used for advancing the language. If you really have an old codebase that cannot be altered for some reason, you could keep using an old version of PHP. I'd rather they kept 5.4 around forever as a long term support version than doubled the complexity of developing PHP as a language. The breaking changes and deprecations with each new version are relatively minor, if those cannot be made to your existing codebase, you also likely wouldn't develop anything new there and wouldn't need a quirks/strict mode. If you really need to, you can run multiple different versions of PHP at the same time.