how to POST from function and "read" return value

Hi,
I know how to make a form that POST, no problem.
We are using this company for sending sms messages from our site. For this I have to POST several variables to a php page on their server (I use php too). This page will return a code, depending on success or errror, and if the latter what error

So in my included "functions" file, I want to create a function to call that will post by itself to this external form, and then return the value returned by the form.

Can this be done, and if so how (yeah yea, i'm a noob)