未知栏＆＃34;＆＃39;＆＃39;在＆＃39;字段列表＆＃39;

Question

我是PHP的新手，无法通过检查类似的问题来解决这个问题。

    <?php
 function charAt($str, $pos)
    {
      return $str{$pos};
    }

    $servername = "localhost";
    $username = 'root';
    $password = '';
    $dbname = 'shop';
    $conn = new mysqli($servername, $username, $password, $dbname)
    or die ("Unable to connect");

    $c_number = $_POST['c_number'];
    $c_name = $_POST['c_name'];
    $c_surname = $_POST['c_surname'];
    $c_address = $_POST['c_address'];
    $c_tnumber = $_POST['c_tnumber'];

    $sql = "UPDATE customers SET c_name = ". $c_name . " WHERE c_number = " . $c_number;

    if ($conn->query($sql) === TRUE) {
        echo "Record updated successfully";
    } else {
        echo "Error updating record: " . $conn->error;
    }

    $conn->close(); 
    ?>

我收到的消息是： 更新记录时出错：未知栏＆＃39;布莱恩＆＃39;在＆＃39;字段列表＆＃39;

如果我这样写的话，效果很好：

$sql = "UPDATE customers SET c_name = 'Bryan' WHERE c_number = " . $c_number;

提前谢谢