convert unix timestamp to a c# 18 digit using php

Question

I am trying to to convert time() in php to a c# equivalent. I am sending a timestamp through web service for authentication and I get a fault with their timestamp displayed in C# 18 digit(633963155279823710 )and my unix time displayed as (1260718725).

Does anyone have php code that will convert the unix time to the c# ticks. In another post I read about using the gmp library? but that is not available on my server.