问PHP复选框问题(未删除行)

EN

    <form method="post" action = "del.php">
    <table border = "1">
        <tr>
            <th>Employee Name</th>
        </tr>
        <?php
            $servername = "localhost";
            $username = "root";
            $password = "";
            $dbname = "lavoratori";

            // Create connection
            $conn = new mysqli($servername, $username, $password, $dbname);

            // Check connection
            if ($conn->connect_error) {
                die("Connection failed: " . $conn->connect_error);
            } 

            $sql = "SELECT nome, id FROM operai";

            $result = $conn->query($sql);

            if ($result->num_rows > 0) {
                // output data of each row
                while($row = $result->fetch_assoc()) {

                    echo"<tr>";
                    echo'<td><input type = "checkbox" name = checkbox[]" value = '.$row['id']."<td>".$row['nome']."</td>";
                    echo"</tr>";  

                }
                echo"</table>";
            }
            $conn->close();
        ?>
        <input type = "submit" name = "delete" id = "delete" value = "Delete Records">
    </form>

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "lavoratori";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
             die("Connection failed: " . $conn->connect_error);
} 

if(isset($_POST['delete'])){
    $chkarr = $_POST['checkbox'];
    foreach($chkarr as $id){
        $sql = "DELETE FROM operai WHERE id = .$id.";
        $result = $conn->query($sql);
    }
    header("Location: /test_purpose/home.php");
}
$conn->close();