Prepared statements in DataTables - php

Question

I want to use prepared statements inside DataTables for the code stated below. It has two queries (query and query1).
Query has two "where conditions" which variables are $classid and $searchfield.
Query1 is a "Limit" statement and also has two variables $start and $end.
The results of both queries should be stored in the $data array. At the moment I don't get any results. How do I bind the paramteres correctly in this case and store the results in the array so DataTables gets a working json?
My try:
$connect = mysqli_connect("localhost", "xxxx", "xxx", "xxx");
$columns = array('itemOverview.id', 'itemOverview.Display_lang');
// Variables to bind
$classid = 4;
$searchfield = $_POST["is_searchfield"];
$start = $_POST['start'];
$end = $_POST['length'];
$query = "SELECT * FROM itemOverview INNER JOIN item ON item.id = itemOverview.id";
$query.= " WHERE item.ClassID = ? AND ";
if(!empty($searchfield)) {
$query .= "itemOverview.Display_lang = ? AND ";
}
$query1 = '';
if($_POST["length"] != -1) {
$query1 = 'LIMIT ?, ?';
}
$stmt = mysqli_stmt_init($connect);
if (!mysqli_stmt_prepare($stmt, $query. $query1)) {
echo "SQL Failed";
} else {
// bind paramaters to the placeholder
mysqli_stmt_bind_param($stmt, "ssss", $classid, $searchfield, $start, $end);
// run parameters inside database
mysqli_stmt_execute($stmt);
$result = mysqli_stmt_get_result($stmt);
$data = array();
while($row = mysqli_fetch_array($result))
{
$sub_array = array();
$sub_array[] = $row["id"];
$sub_array[] = $row["Display_lang"];
$data[] = $sub_array;
}
}
$output = array(
"draw" => intval($_POST["draw"]),
"data" => $data
);
echo json_encode($output);