为什么这个PHP代码if-else条件不起作用

在if (!empty) {}部分之后检查IF ELSE语句。

<form action="first.php" method ="post">
 <input type="text" name="username" >Username</br>
 <input type="password" name="password" >Password</br>
 <input type="submit"  value="Submit"></br>
</form>
<?php
if (isset($_POST['username'] )&& isset($_POST['password'])) {
    if (!empty($user) && !empty($pass)) {
        $user = $_POST['username'];
        $pass = $_POST['password'];
        if ($user ==="habib"  && $pass === "1234") {
            echo $user;
            echo $pass;
        } else {
            echo "wrong password and username";
        }
    }
}
?>