php简单的判断用户登陆

<form action="test.php" method="post" enctype="multipart/form-data">
    <input type="text" name="username"><br>
    <input type="password" name="password"><br>
    <input type="submit" value="提交">
</form>

<?php
//$res=[];
//$res[‘username‘]=$_POST[‘username‘];
//$res[‘password‘]=$_POST[‘password‘];
//var_dump($res);
$username = $_POST[‘username‘];
$password = $_POST[‘password‘];

if ($username == ‘‘ || is_null($username || empty($username))) {
    echo ‘姓名不能为空‘;
} elseif ($password == ‘‘ || is_null($password || empty($password))) {
    echo ‘密码不能为空‘;
} else {
    echo ‘a‘;
}