如何使用用户登录信息在PHP和MySQL查询中创建IF ELSE条件

<?php
// 获取用户登录信息
$username = $_POST['username'];
$password = $_POST['password'];

// 连接MySQL数据库
$servername = "localhost";
$dbusername = "root";
$dbpassword = "password";
$dbname = "mydb";

$conn = new mysqli($servername, $dbusername, $dbpassword, $dbname);

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

// 创建查询语句
$sql = "SELECT * FROM users WHERE username='$username' AND password='$password'";

// 执行查询
$result = $conn->query($sql);

// 检查查询结果
if ($result->num_rows > 0) {
    // 用户登录成功
    while($row = $result->fetch_assoc()) {
        echo "欢迎, " . $row["username"];
    }
} else {
    // 用户登录失败
    echo "用户名或密码错误";
}

// 关闭数据库连接
$conn->close();
?>