Ask user input symbolic variables for GUI application

I am creating a simple GUI . In one part (for the user who has symbolic toolbox) to input a symbolic variable and function. So when you are writing

if true
  syms x F   % want to be able to ask user input x,y,z etc
F=get(handles.input,'String')
Sym_Int=eval(int(F,x,L,R));   % I put eval there because some funny reason when 
                            % sin(x) I get (L=0, R=1) 1-cos(1) not 0.4596..                
set(handles.result,'String',Sym_Int)
end

For that I have a edit box where I am asking user input the variable "x" or whatever they want. According to that I need to update syms ? F=sin(?)

is there a way of doing that?