Looks like you are talking about C/C++ code. If that is the case, I am confused. How could:
ever work? That should not work for one of two reasons: either a should be a pointer to a character array or mystr a single character.
That being said, there are some other issues with your code. You declare prhs to be a pointer to a const mxArray. Then you try to assign values to prh, something that you have never declared. Finally you are trying to assign a character and a numeric value to prh, which is a pointer to an mxArray. That's a no-no.
Anyway, knowing what errors you get and what you are actually trying to do would greatly facilitate the task of those trying to help you.