Correct way to update $_SESSION username in table - php

Question

I've updated the post since I made a bit of change thanks to #user3282898! Though I still can't push the update to the DB.
The table column $id, $issue, $last_mod has already an existing content, I just need to update the content of issue andlast_modcolumn with respect to its$id`.
Here's what I have so far:
<?php
session_start();
session_regenerate_id();
if(!isset($_SESSION['username']))
{
header("Location: login.php");
}
?>
<?php
$conn = mysqli_connect("localhost", "root", "", "order");
if (isset($_GET['id']) && is_numeric($_GET['id']))
{
//id value
$id = $_GET['id'];
$last_mod = $_SESSION['username'];
mysqli_query($conn, "UPDATE order.coupon SET issue='Resolved', last_mod=".$last_mod." WHERE id=".$_POST['id']) //update won't work
or die(mysqli_error());
header("Location: form.php");
}
else
{
header("Location: form.php");
}
?>
I've tried omitting the $last_mod to isolate the issue of updating and find that this statement works:
mysqli_query($conn, "UPDATE order.coupon SET issue='Resolved' WHERE id=$id")
However it won't work with $last_mod in it:
mysqli_query($conn, "UPDATE order.coupon SET issue='Resolved', last_mod=".$last_mod." WHERE id=".$_POST['id'])
or
mysqli_query($conn, "UPDATE order.coupon SET issue='Resolved', last_mod=".$last_mod." WHERE job_id=$job_id")
Your suggestion/opinion is always welcome, thanks in advance guys!

Answer 1

$_SESSION['username']='$last_mod'
You are updating the $_SESSION['username'] field in your table which does not exist!
The $_SESSION['username'] is the username and the column to be updated is the last_mod.
You should do this as i said in my comment:
UPDATE order.coupon SET last_mod='$last_mod' WHERE id='".$_POST['id']."'");

Answer 2

This should work for you.
$last_mod = $_SESSION['username'];
$query = "INSERT INTO coupon(last_mod)
VALUES ($last_mod)";

Answer 3

that query does not seem to make any sense; so how about ...
$sql = "UPDATE `coupon` SET `last_mod` = NOW() WHERE `coupon_id` = ?";
because it appears, as if you were trying to insert a username as the last_mod (last modification) timestamp of a coupon. just add a die($sql); whenever being uncertain why generated SQL won't work. besides, using the user_id (or the coupon_id) instead of the username would be suggested; because indexed INT fields are way quicker to query by.