Ugh, I got that question in an interview. Only they wanted me to write up a basic arbitrary precision math library, in C, on paper. (I may have chosen to do it on paper.) It was harder than I expected. I didn't do that well and didn't manage to finish in the time allotted -- halfway through, I switched from encoding things as ASCII numerals to base-256 binary (or the reverse?) -- but it was enough that the interviewer said it was clear I understood what was required. I got the job.

I thought it was a hard but fair question. I still feel bad about not doing well. It wasn't all that close to what I'd be doing in the position, but it wasn't unrelated. Perhaps if it were a PHP role I'd be more bothered? But even there, it seems like one way to check if someone understands what's going on under the hood, so as to not naively do expensive stuff without realizing it. The fact that one should not write their own library, and that if you tried you should get code reviewed to hell, isn't relevant if that's the purpose.