You're looking for makevar. Avoid using 'var' by itself as a variable name.
I.e.
PHP Code:
function setVar(vari, value) {
makevar(vari) = value;
}
then in your code when the button is clicked..
PHP Code:
function Dev_Client_OK.onAction(){
setVar(Dev_Client_Var.text, Dev_Client_NewVar.text);
}