PDOStatement :: fetch_array ()

I have a problem with a% of type Select (follow the example below). I am using version 7.0 of PHP and as PostgreSQL DBMS. I tried to use the input function but it was unsuccessful, does anyone have any idea what might be wrong?

<?php$consulta="SELECT * FROM atividades";
    $query = $db->db->query("SELECT * FROM atividades");
?>
 <select>
          <?php 
            echo "<select name='descricao'>";
              while($cont = $query->fetch_array()) {
                echo "<option value='".$cont["id"]."'>".$cont["descricao"]."</option>";
              }
            echo "</select>";
          ?>
      </select>

This is the return specified by var_dump ($ query);