If all your PHP errors and warnings suddenly disappear from your IDE editor window, despite you having the language level configured correctly etc., double check you haven’t turned on Reader Mode by accident, as by default it hides “error and warning highlighting and the inspection widget”.
Therefore you’ll see the message “No errors found in this file” when pressing F2, even though there are some.
Also by default, it turns on font ligatures (the fancier versions of =>, != etc.) so if you’re seeing those and wondering why, even though you have the boxes unticked in Preferences Editor > Font, or Editor > Color Scheme > Color Scheme Font / Console Font – again, check it’s not activated.
All this is configurable in Preferences > Editor > Reader Mode.
Note that Reader Mode is toggled on and off for individual files.