php - MySQL无法执行

Question

我是编程新手，正在尝试使用 php/mysql 创建登录表单，但遇到了一些问题。我无法理解为什么我的 sql 代码无法执行，我使用的是本地主机。你们能帮帮我吗？谢谢这是我写的代码

<?php

$host = "localhost";
$user = "root";
$pass = "";
$db = "login";

mysql_connect($host, $user, $pass);
mysql_select_db($db);


if (isset($_POST['username'])){
    $username = $_POST['username'];
    $password = $_POST['password'];

    $sql = "SELECT * FROM users WHERE username'".$username."' AND password='".$password."' LIMIT 1";
    $res = mysql_query($sql);
    if (mysql_num_rows($res) == 1){
        echo "You have successfully logged in.";
        exit();
    }else{
        echo "Invalid password information.";
        exit();
    }
}
?>

Answer 1

我想你忘记了 =

 $sql = "SELECT * FROM users WHERE username='".$username."' AND password='".$password."' LIMIT 1";