Aca te dejo un ejemplo con una base de datos incluida.
<?php
    $db = [
        'server' => 'localhost',
        'user' => 'root',
        'password' => '',
        'database' => 'test'
    ];
    $users = [];
    $datos = [];
    $mysqli = new mysqli(
        $db['server'],
        $db['user'],
        $db['password'],
        $db['database']
    );
    if(!$db)
        die('imposible conectar a la base de datos');
    if(isset($_POST['user']))
    {
        $user_id = (int)$_POST['user'];
        if(empty($user_id))
            die('usuario no ingresado');
        $result = $mysqli->query("SELECT username FROM usuarios WHERE id = $user_id LIMIT 1");
        $user_exists = ($result->num_rows > 0);
        $user = $result->fetch_array(MYSQLI_ASSOC);
        if(!$user_exists)
            die('usuario no existente');
        $datos['username'] = $user['username'];
        $result->free_result();
        $result = $mysqli->query("SELECT * FROM datos WHERE user_id = $user_id LIMIT 1");
        $datos += $result->fetch_array(MYSQLI_ASSOC);
        $result->free_result();
    }
    else
    {
        $result = $mysqli->query("SELECT id, username FROM usuarios");
        if(!$result) {
            die('hubo un error [' . $mysqli->error . ']');
        }
        while($row = $result->fetch_assoc())
        {
            $users[] = $row;
        }
        $result->free_result();
    }
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>select list</title>
</head>
<body>
<?php
    if(!empty($datos)) {
        echo 'usuario: <strong>' . $datos['username'] . '</strong><br>';
        echo '<table>
            <tr>
                <th>nombre</th>
                <th>apellido</th>
                <th>edad</th>
                <th>lenguajes</th>
            </tr>
            <tr>
                <td>' . $datos['nombre'] . '</td>
                <td>' . $datos['apellido'] . '</td>
                <td>' . $datos['edad'] . '</td>
                <td>' . $datos['lenguaje'] . '</td>
            </tr>
        </table>';
    }
    else {
        echo '<form method="POST" action="">
        <select name="user">';
            foreach($users as $user)
                echo '<option value="' . $user['id'] . '">' . $user['username'] . '</option>';
        echo '</select><br>
        <input type="submit" value="enviar">
        </form>';
    }
?>
</body>
</html>
configura el array $db
$db = [
    'server' => 'localhost',
    'user' => 'root',
    'password' => '',
    'database' => 'test'
];
https://db.tt/MeUEqaD1