C# SHA-1 vs. PHP SHA-1…Different Results?

I am trying to calculate a SHA-1 Hash from a string, but when I calculate the string using php’s sha1 function I get something different than when I try it in C#. I need C# to calculate the same string as PHP (since the string from php is calculated by a 3rd party that I cannot modify). How can I get C# to generate the same hash as PHP? Thanks!!!