So how would you represent an absence of a value? In an enum that'd be easy, but there is a semantic difference between an empty string and a null string.

Or would you propose sending a boolean to indicate null as well as a string in a data type?

See the Option/Maybe pattern, which uses sum types rather than unions or sentinel values.

https://www.lucidchart.com/techblog/2015/08/31/the-worst-mis...

There is nothing wrong with using null in PHP...