changing assigned name from left side to right side (function input)

Hello

I am trying to automate and make it flexible function inputting from user:

    syms F  var 

    var = input('Variable is : ' , 's');   
    % I want to know which variable user wants to use like x, y, z
    U_test=input('Enter the test function U_test=  '); 
    % and user will input something like t^4+3*t or x^5

in order to this to work I need a command that would var - t allow to use interchangeably. Between two lines.

Anybody have any idea?