$ _POST через jquery-php

Когда я пытаюсь переслать значение $ _post через jquery в php-скрипт, я получаю это уведомление: "Уведомление PHP: неопределенный индекс: id"

Я вижу значение id в инструментах разработчика, например. : 'id: 8519/8520/8521/8522' но на сайте сценарий останавливается на позиции $ _POST ['id'].

вот часть кода:

        <button class = "btn btn-danger" id = "export">export xls</button>

        <form action = "baza_site.php" method = "POST">
                        <input type = "text" class = "form-control" placeholder = "Search" name = "search">
                        <button class = "btn btn-default" type = "submit" name = "submit" >
                        <i class = "glyphicon glyphicon-search"></i>
                        </button>
                        </div>
            </div>          
        </form>

<?php

                    require_once 'config.php';

                    $output = '';
                    $page = isset($_GET['p'])? $_GET['p'] : '';
                    if ($page == 'xls'){
                        $myid = $_POST['id'];
                        $id = str_replace(' ', ',', $myid);

                        $sql = "SELECT * FROM baza_site WHERE id IN($id)";
                        if ($result = $mysqli->query($sql)){
                        if ($result->num_rows > 0)
        {

         $output .= '
            <table border = "1">  
              <tr>  
                <th>id</th>  
                <th>site</th>   
                <th>location type</th>  
                <th>city</th>
                <th>address</th>
                <th>access details</th>
                <th>service area</th>
              </tr>
                    ';
         while($row = mysqli_fetch_array($result))
        {
         $output .= '
                <tr>  
                  <td>'.$row["id"].'</td>  
                  <td>'.$row["site"].'</td>
                  <td>'.$row["locationType"].'</td>  
                  <td>'.$row["city"].'</td>  
                  <td>'.$row["address"].'</td>
                  <td>'.$row["accessDetails"].'</td>
                  <td>'.$row["ServiceArea"].'</td>

                </tr>
                    ';
        }
         $output .= '</table>';

  header('Content-Type: application/vnd.ms-excel');
  header("Content-Disposition: attachment; filename = ".'woclosed'.date("Y-m-d_H_i_s").".xls");
  header("Pragma: no-cache");
  header("Expires: 0");
  echo $output;
 }
                    }}

                    require_once 'config.php';

                    if (isset($_POST["submit"])) {
                    $search = mysqli_real_escape_string($mysqli, trim($_POST['search']));
                    mysqli_set_charset( $mysqli, 'utf8');

                    $sql = "SELECT * FROM baza_site WHERE site LIKE '%$search%' OR city LIKE '%$search%' OR address LIKE '%$search%' OR accessDetails LIKE '%$search%' OR ServiceArea LIKE '%$search%' OR locationType LIKE '%$search%'";
                    if ($result = $mysqli->query($sql)){
                        if ($result->num_rows > 0){
                            $ilosc = mysqli_num_rows($result);
                            echo "<b>ilość rekordów : " .$ilosc."</b>";
                            echo "<table id='myTable' class='tablesorter-blackice'>";
                                echo "<thead>";
                                    echo "<tr>";
                                        echo "<th style='text-align:center;' class='filter-false'><input type='checkbox' id='checkall'/></th>";
                                        echo "<th>site</th>";
                                        echo "<th>location type</th>";
                                        echo "<th>city</th>";
                                        echo "<th>address</th>";
                                        echo "<th>access details</th>";
                                        echo "<th>service area</th>";
                                    echo "</tr>";
                                echo "</thead>";
                                echo "<tbody>";
                                while($row = $result->fetch_array()){
                                    echo "<tr>";
                                        echo "<td class='text-center'>"."<input type='checkbox' name='checkboxlist' class='checkitem' value = ". $row['id']."/> </td>";
                                        echo "<td>" . $row['site']. "</td>";
                                        echo "<td>" . $row['locationType']. "</td>";
                                        echo "<td>" . $row['city'] . "</td>";
                                        echo "<td>" . $row['address'] . "</td>";
                                        echo "<td>" . $row['accessDetails'] . "</td>";
                                        echo "<td>" . $row['ServiceArea'] . "</td>";


                                    echo "</tr>";
                                }
                                echo "</tbody>";                            
                            echo "</table>";
                            // Free result set
                            $result->free();
                        } else{
                            echo "<p class='lead'><em>Brak informacji w bazie.</em></p>";
                        }
                    } else{
                        echo "ERROR: Could not able to execute $sql. " . $mysqli->error;
                    }
                    }
                    // Close connection
                    $mysqli->close();
                    ?>  



    </div>
    </div>
                        <script type = "text/javascript">

            $("#myTable").tablesorter({
    headers: {
      0: { sorter: false, parser: false }
    }
});

<script type = "text/javascript">

        $('#checkall').change(function(){
            $('.checkitem').prop("checked", $(this).prop("checked"))
        })

        $('#export').click(function(){
            var id = $('.checkitem:checked').map(function(){
                return $(this).val()
            }).get().join(' ')
            $.post('baza_site.php?p=xls', {id : id})
            });

</script>