How to establish a PHP database connection from scratch

$servername = "localhost";
$username = "root";
$password = "mypassword";
$dbname = "mydatabase";

try {
  // 创建一个新的 mysqli 连接对象
  $conn = new mysqli($servername, $username, $password, $dbname);

  // 检查连接是否成功
  if ($conn->connect_error) {
    throw new Exception($conn->connect_error);
  }

  // 执行查询
  $result = $conn->query("SELECT * FROM mytable");

  // 处理查询结果
  while ($row = $result->fetch_assoc()) {
    echo $row['id'] . " - " . $row['name'] . "<br>";
  }

  // 关闭连接
  $conn->close();
} catch (Exception $e) {
  echo $e->getMessage();
}  

try {
  // 创建一个新的 PDO 连接对象
  $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);

  // 设置 PDO 错误模式
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

  // 执行查询
  $stmt = $conn->prepare("SELECT * FROM mytable");
  $stmt->execute();

  // 处理查询结果
  while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    echo $row['id'] . " - " . $row['name'] . "<br>";
  }

  // 关闭连接
  $conn = null;
} catch (PDOException $e) {
  echo $e->getMessage();
}  

// 使用 mysqli 扩展
$conn = new mysqli("localhost", "root", "mypassword", "mydatabase");

if ($conn->connect_error) {
  die("连接失败: " . $conn->connect_error);
}

// 执行查询
$result = $conn->query("SELECT * FROM users");

// 获取查询结果
$users = $result->fetch_all(MYSQLI_ASSOC);

// 循环遍历结果
foreach ($users as $user) {
  echo $user['id'] . " - " . $user['username'] . "<br>";
}