Also enforce more rigorous Excel Function implementation by testing the underlying implementation, call via the Calc Engine, and execution from in a worksheet.
Separate out unhappy path (exception) checks into a separate test, so that a single test isn't made overcomplex checking for every potentiality.
Scrutinizer may dislike variadics, for variable number of arguments of mixed type; but tough. It's 100% valid PHP, accepted by phpstan, and makes life a lot easier.
Initial work here covers all the database and datetime unit tests for Excel function implementations.
MarkBaker