php和mysql登录程序

<?php
// 数据库连接配置
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "myDB";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

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

// 获取表单提交的用户名和密码
$username = $_POST['username'];
$password = $_POST['password'];

// 查询数据库
$sql = "SELECT * FROM users WHERE username='$username' AND password=MD5('$password')";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 用户名和密码匹配
    echo "登录成功";
    // 设置会话
    session_start();
    $_SESSION['username'] = $username;
} else {
    echo "用户名或密码错误";
}

$conn->close();
?>