获取$ _POST中所选项的值

i have a dropdownthat fill dynamic with ajax,

<select id="singer_list">
  <option value='.$value.'>'.$option.'</option>
    </select>

my problem is: i can't get a Value of dropdown on the $_POST i'm using:

$singer_name=$_POST["singer_list"];
echo $singer_name;

but $singer_name is show nothing! how to select value of dropdown in $_POST?

my ajax code is :

<script type="text/javascript">
$(function(){
    $("#genre_list").change(function(){
        var genre_id=$("#genre_list").val();
        $.ajax({
             type:'POST',
             url:'ajax_singer_list.php',
             data:{'genre_id':genre_id},
             success:(function(data){
                     $("#singer_list").append(data);
                 })

            })
        })
    })
</script>

ajax_singer_list.php handler:

<?php
include('../db_inc.php');
$genre_id =$_POST['genre_id'];
$result = $connection->query("SELECT singer_name,singerid from singers INNER JOIN genre_singer ON singers.singerid=genre_singer.f_singer_id where f_genre_id = '$genre_id'")or die($connection->error);
        while($row = $result->fetch_object()){
            $singer_name = $row->singer_name;
            $singer_id = $row->singerid;
            echo "<option value=$singer_id>$singer_name</option>";
            }
            $connection->close;
?>

Apart from if you actually post etc, the first thing is:
if you want to post a <select>, you need to give it a name, not just an 'id'.

 <select id="singer_list" name="singer_list" >