$id = $_POST["id"];
    if(!is_string($id) && !empty($id)) {
        $delete = mysqli_query($connect,"DELETE from users WHERE ID=$id");
        $_SESSION["succMsg"] = "User have been successfully removed";
        Header("Location:login_update.php");
    } else {
        $_SESSION["succMsg"] = "Unable to execute the query.";
        Header("Location:login_update.php");
    }

$id = (int) $_POST["id"];

if (isset($_POST["id"])) {
    $id = (int) $_POST["id"];
    // Do your query....
}

function addOptions()
{
    global $connect;
    $result = mysqli_query($connect,"SELECT * FROM users ORDER BY ID ASC");
    while ($row = mysqli_fetch_assoc($result)) {
        $id = $row["id"];
        echo "<option value='$id'>$id</option>";
    }
}

function addOptions()
{
    global $connect;
    $result = mysqli_query($connect,"SELECT * FROM users ORDER BY ID ASC");
    while ($row = mysqli_fetch_assoc($result)) {
        $id = $row["id"];
        echo "<option value="".$id."">$id</option>";
    }
}