mysqli怎么连接数据库

<?php
$host = 'localhost';
$user = 'username';
$password = 'password';
$database = 'database_name';

// 创建连接
$conn = mysqli_connect($host, $user, $password, $database);

// 检查连接
if (!$conn) {
    die('Connection failed: ' . mysqli_connect_error());
}

echo 'Connected successfully';

// 关闭连接
mysqli_close($conn);
?>

<?php
$host = 'localhost';
$user = 'username';
$password = 'password';
$database = 'database_name';

// 创建连接
$conn = new mysqli($host, $user, $password, $database);

// 检查连接
if ($conn->connect_error) {
    die('Connection failed: ' . $conn->connect_error);
}

echo 'Connected successfully';

// 关闭连接
$conn->close();
?>

$stmt = $conn->prepare("SELECT * FROM users WHERE username = ?");
$stmt->bind_param("s", $username);
$username = $_POST['username'];
$stmt->execute();
$result = $stmt->get_result();