$_SESSION not working - php

Question

So i am not sure what to do anymore.
I've been trying to create a register/login system for my website. After a lot of struggeling my register now works but i can't yet login to it. I am pretty sure it is a $_session related problem.
So I have two files, one called get_users.php (i know it's a bad name) and one called cart.php. Neither of them has whitespace at the start.
What am i actually trying to do? I am trying to get my session to show up on cart.php.
get_users.php:
<?php
$username = $_POST['username'];
$password = $_POST['password'];
$con = new mysqli("localhost","root","","ismsite");
$query = ("SELECT name, comment FROM comments ORDER BY id DESC");
$result = mysqli_query($con, $sql, MYSQLI_BOTH);
session_start();
$_SESSION["user_id"] = $row["user_id"];
header('Location: cart.php');
exit();
?>
and at the start of cart.php
<?php
session_start();
include 'config/config.php';
echo $_SESSION["user_id"];
?>
I really am at my wits end here. I've searched this site but i could not find a solution to my problem. Anyone who knows what the problem is?
Additional info:
-Latest php installed
-I am running it on a virtual webserver that runs the latest ubuntu client with LAMP stack installed.
-Database works just fine
Thanks in advance
EDIT:
I changed $row["user_id"]; to $result["user_id"];
But it still doesn't show up

Answer 1

Try this
// Define $username and $password
$username=$_POST['username'];
$password=$_POST['password'];
//for hashing passwords
$username = stripslashes($username);
$password = stripslashes($password);
$username = mysqli_real_escape_string($db,$username);
$password = mysqli_real_escape_string($db,$password);
$password = md5($password);
//Check username and password from database
$sql="SELECT userid FROM users WHERE username='$username' and password='$password'";
$result=mysqli_query($db,$sql)
or die("Error");
$row=mysqli_fetch_array($result,MYSQLI_ASSOC);
//If username and password exist in our database then create a session.
//Otherwise echo error.
if(mysqli_num_rows($result) == 1)
{
$_SESSION['username'] = $username; // Initializing Session
}else
{
$error = "Incorrect username or password.";
}
But you will have to tweak according to your table structure.

Answer 2

May be $_SESSION["user_id"] not set. You can test $_SESSION array by
print_r($_SESSION) or print_r($GLOBALS);