Valor da lista suspensa dinâmica do banco de dados

<form id="form1" name="form1" method="post" action="<?php $_SERVER['PHP_SELF']?>">
 Drop1
 <?php
    $mysqli = new mysqli("localhost", "root", "", "lists");
    $result = $mysqli->query("SELECT * FROM main_list GROUP BY id ORDER BY id");
    $option = '';
    while ($row = $result->fetch_assoc()) {
       $option .= '<option value = "'.$row['value'].'">'.$row['value'].'</option>';
    }
    ?>

     <select id="main" name="main">
     <option selected=selected>Choose</option>
       <?php echo $option; ?>
     </select>
    <div id="sublist"></div>

   <input type="submit" name="submit" value="Submit" />
</form> 

    $('#main').change(function(){
        $.ajax({
        url : 'secondlist.php',
        data :{mainlist_id : $(this).val()},
        dataType:'html',
        type:'POST',
        success:function(data){
         $('#sublist').html(data);
        }
      });
   });

    $mysqli = new mysqli("localhost", "root", "", "lists");
    $result1 = $mysqli->query("SELECT * FROM sub_list as s, main_list as m where s.category_id = m.id and s.category_id = $_POST['mainlist_id'] GROUP BY value ORDER BY id ");
    $option1 = '';

    while ($row = $result1->fetch_assoc()) {
        $option1 .= '<option value = "'.$row['value'].'">'.$row['value'].'</option>';
    }

    $output = 'Drop2 ';
    $output .= '<select name="sub" id="sub">';
    $output .= '<option value=" " disabled="disabled" selected="selected">Choose one</option>';
    $output .= $option1;
    $output .= '</select> ';
echo $output;
exit;
?>