Как выводить имя пользователя по id, на главную страницу в админке?

session_start();
if(isset($_POST['login_in']))
{
    $query = mysqli_query($mysql,"SELECT * FROM `user` where email= '$_POST[email]' and password ='$_POST[password]'");
    if(mysqli_num_rows($query))
    {
        $_SESSION['register'] = $_POST['email'];
        $_SESSION['password'] = $_POST['password'];

        echo "<script>document.location.href = 'index.php'</script>";
    }
    else
    {
        echo "<script>document.location.href = 'register.php'</script>";
    }
}

<?
session_start();
            include "../db.php";


            if(isset($_POST['sign_up'])){
                $insert = mysqli_query($mysql, "INSERT INTO `user`(`user_id`, `name`, `surename`, `email`,`password`, `number` ) VALUES ( '$_POST[user_name]','$_POST[sure_name]','$_POST[email]','$_POST[password]','$_POST[phone]')");

                if ($insert) {
                    echo "<script>document.location.href = 'login.php'</script>";
                }
            }

            ?>