my function is not working

                        function username_exists($username){
                            global $db;
                            $sql = "SELECT 'username' FROM users WHERE 'username' = '$username'";
                            $result = mysqli_query($db, $sql);
                            confirmquery( $result );

                             if (mysqli_num_rows($result) > 0) {
                               return true;
                             }else {
                               return false;
                             }
                        }
                        if(username_exists($username)){
                                $error['username'] = 'Username already exists';
                              }