Ask HN: Is saying "I don't know during an interview" something negative?

Hello,

I just interviewed for a position doing PHP/codeigniter. One of the technical questions I was asked had me working with multi-dimensional arrays. This is an area where I have not had much need to work with in the past. I could not complete the whole exercise. I was honest and did not cheat by asking for a solution at some web forum. I did not get the job due to not being able to do this.

Did I make a mistake by being honest and should have just googled for the answer?

Please give me your insight because I'm just confused by the whole thing.