php - How can I get an unknown username given an ID?

Solution:

I would strongly recommend that you avoid the mysqli extension. You should use some database abstraction library instead of using the mysqli functions directly; the mysqli class is not suited to be used on its own.

If you really want to do it purely with the mysqli class, you have few options.

First, you need to open the connection properly. These 3 lines ensure you have the connection ready:

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT); // enable error reporting
$mysqli = new mysqli('localhost', 'username', 'password', 'dbname');
$mysqli->set_charset('utf8mb4'); // always set the charset
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Then you need to prepare a statement, bind the parameter and execute it.

$id = '10527391670258314752';
$stmt = $mysqli->prepare('SELECT username FROM user_data WHERE id=?');
$stmt->bind_param('s', $id);
$stmt->execute();
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Once the statement is executed you need to fetch the results. If you have only one variable you could simply usebind_result()

$stmt->bind_result($_SESSION['name']);
$stmt->fetch();
echo $_SESSION['name'];
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However, this approach is not recommended and not very flexible. Instead it's better to fetch the whole result set and get an array containing the first row.

$result = $stmt->get_result();
$row = $result->fetch_array();
$_SESSION['name'] = $row['username'];
echo $_SESSION['name'];
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As an alternative with PDO the same code would look like this:

session_start();

$pdo = new PDO("mysql:host=$host;dbname=$db;charset=$charset", $user, $pass, [
    \PDO::ATTR_ERRMODE => \PDO::ERRMODE_EXCEPTION,
    \PDO::ATTR_EMULATE_PREPARES => false,
]);

$id = '10527391670258314752';

$stmt = $pdo->prepare('SELECT username FROM user_data WHERE id=?');
$stmt->execute([$id]);
$_SESSION['name'] = $stmt->fetchColumn();
echo $_SESSION['name'];
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