<?php
    if(isset($_POST['username'])) {
        $user = mysqli_fetch_row(mysqli_query($db, "SELECT * FROM srudentlist WHERE username = '" . $_POST['username'] . "'"));

        if($user != null) {
            $_SESSION['user_id'] = $user['id'];
            $_SESSION['fio'] = $user['fio'];
        }
    }
?>