put MYSQL data in the combobox of my form using class and php

I want to load COMBOBOX from the user registration maintenance form into the user's NAME data from MYSQL. I have a method below that does the query in the mysql database and returns an array

public function LerDadosUsu(){
    try{
        $lerusu = "SELECT * FROM usuario ";
        $listusu = $this->con->Connect()->prepare($lerusu);
        $listusu->execute();
        $retDados = $listusu->fetchAll(PDO::FETCH_ASSOC);
        return $retDados;
        }
     catch(PDOException $erro_2){
     echo 'erro'.$erro_2->getMessage();
        }

    }

I have the form below that I want to show this information, but where is the option value of combobox does not bring the information that already exists in mysql

      <form method="post" name="frm_usuario">

                      <div class="form-group">
                                          <?php    

                                         echo '<label for="filter">Filtrar Usuário</label>

                                         <div class="input-group"><span class="input-group-addon"><i class="fa fa-search"></i></span><select class="form-control">
                                         /*crio o objeto para buscar valor*/
                                         $objeto = new Usuario();
                                         $usuario = $objeto->LerDadosUsu();

                                         /*crio um foreach para retornar o campo*/   
                                         foreach($usuario as $usuario):

<option value="<?$usuario['.nom_usuario.'];?>"selected>Escolha o usuario</option>
                                         endforeach;
                                         </select>
                                        ?>

                       </div>
                       </div>

The error that appears on my form when I'm running is this below

ERROR: Use of undefined constant user_name - assumed 'user_name' in C: .. \ user.php on line 327