php calling function from another file using button click and ajax [duplicate]

i have a functions.php page with a dologout function

function doLogout() {
    if (isset($_SESSION['user_id'])) {
        unset($_SESSION['user_id']);
        unset($_SESSION['location']);
    }
    header('Location: index.php');
    exit;
}

and a home.php page with a logout button with onclick method to logout function which is as follow

function logout()
                            { 
                              $.ajax({
                              url: "functions.php",
                              type: "POST",
                              dataType: "dataString",
                              data: {func:"doLogout"},
                              }).done(function() {
                              alert( "LoggedOut Succesfully");
                               }); 
                                }
                              }
                       }

But this function is not working i tried looking at other answers but none of them actually answers how to call a function using ajax. Can anyone please help me? Thanks in advance