Algorithmic complexity of PHP function strlen()
O(1) since the length is stored as an attribute: source
However, this trivia is worth countering with a discussion about micro-optimising theatre, as kindly provided by our hosts here and here; read those two links and you'll find a good talking point to change the momentum of the conversation next time similar questions come up, regardless of whether you know the particular answer!
How the interviewer reacts to your tangent will tell you a lot about how much you want to work with them..
I would assume that function is O(n) because it would need to iterate through the string once.