MySQL UPDATE查询问题

I have a contact saved in database which I am using the code below to update. However, it's not being updated. I have already checked my POST values to make sure they are not empty. I appreciate any guidance.

$conn=mysqli_connect("localhost","root", "", "contacts");
$sql = "UPDATE contact SET cont_fname='".$_POST['cont_fname']."', cont_family='".$_POST['cont_family']."', cont_phone='".$_POST['cont_phone']."' WHERE cont_id='".$_POST['cont_id']."'";
$add_contact = mysqli_query($conn, $sql) or die(mysqli_error($conn));

Use this

   $cont_fname =  $_POST['cont_fname'];
   $cont_family =  $_POST['cont_family'];
   $cont_phone =  $_POST['cont_phone'];
   $cont_id =  $_POST['cont_id'];

    $mysqli = new mysqli('localhost','root','','contacts');

    if ($mysqli->connect_error) 
    {
        die('Error : ('. $mysqli->connect_errno .') '. $mysqli->connect_error);
    }

    //MySqli Update Query
    $results = $mysqli->query("UPDATE contact SET cont_fname= '$cont_fname', cont_family ='$cont_family', cont_phone='$cont_phone' WHERE cont_id='$cont_id'");


    if($results)
    {
        print 'Success! record updated / deleted';
    }
    else
    {
        print 'Error : ('. $mysqli->errno .') '. $mysqli->error;
    }

Alternative 1:

$sql = "UPDATE contact SET cont_fname='".$_POST[cont_fname]."', 
    cont_family='".$_POST[cont_family]."', cont_phone='".$_POST[cont_phone]."' 
    WHERE cont_id='".$_POST[cont_id]."'";

Alternative 2:

$cont_fname = $_POST['cont_fname'];
$cont_family = $_POST['cont_family'];
$cont_phone = $_POST['cont_phone'];
$cont_id = $_POST['cont_id'];
$sql = "UPDATE contact SET cont_fname='$cont_fname', 
    cont_family='$cont_family', cont_phone='$cont_phone' WHERE cont_id='$cont_id'";