Use of global variables

I’m a bit lost with global variables.
I use [usr_login] and another [usr_language] in a few forms and this works fine. Both are set at the login form (I added the second one myself).
I have a form which is openend by use of a button from another form and now these globals are empty. What do I do wrong here? I don’t use the variables in fields of the first form. I don’t reset anything. They should be filled with the same data, right?

Once they are filled they stay indeed. As long as you are using the same php session id… (check your php session id in your browser).