[5.4] Just use is_object()

[KennedyTedesco]

Ref: #19838

[Douglasdc3]

Good fix although the actual change (adding support for objects) is a breaking change see

[KennedyTedesco]

@Douglasdc3 Maybe we can mitigate by evaluating if the __toString() is implemented? See the new commit.

FYI: I'm just trying to fix it, I haven't a use case for it.