<?php
// enable error checking for development
ini_set('display_errors',1); // enable php error display for easy trouble shooting
error_reporting(E_ALL); // set error display to all

// use standard naming convention
// example: $result // good
// $run_query // bad

$query = "SELECT SUM(REPLACE(CONCAT(`sub_total`), ',', '')) AS total FROM shopping_cart WHERE `user_id` = {$_SESSION['user_id']}";
$result = mysqli_query($conn,$query) or die(mysqli_error($conn));

while ($row = mysqli_fetch_assoc($result))
{
    print_r($row['total']);
}
?>