The Filename of the image I'm uploading is not changing - php

Question

Evertime I upload a picture, the FILENAME is NOT CHANGING the static value(filename) that is inserting in database is always "0.png" I don't know how is that happening, Please Help me how to fix this problem.
Here is my code:
<?php
session_start();
include("../db_connection.php");
$seller_id = $_SESSION['seller_id'];
$trade_name = $_POST ['trade_name'];
$s_address = $_POST ['s_address'];
$opening_time = $_POST ['opening_time'];
$opening_days = $_POST ['opening_days'];
$order_cutoff = $_POST ['order_cutoff'];
$seller_delivery_time = $_POST ['seller_delivery_time'];
$area_covered_delivery = $_POST ['area_covered_delivery'];
$delivery_fee = $_POST ['delivery_fee'];
$extension = pathinfo($_FILES['s_image']['name'], PATHINFO_EXTENSION);
$sql = mysqli_query($db, "UPDATE selling_details
SET
opening_time = '$opening_time',
opening_days = '$opening_days',
order_cutoff = '$order_cutoff',
seller_delivery_time = '$seller_delivery_time',
area_covered_delivery = '$area_covered_delivery',
delivery_fee = '$delivery_fee'
WHERE seller_id= '" . $_SESSION['seller_id'] . "' ");
if ($sql)
{
$id = mysqli_insert_id($db);
$filename = $id.'.'.$extension;
if(move_uploaded_file($_FILES['s_image']['tmp_name'], 'upload/'.$filename))
{
$sql2 = mysqli_query($db, "UPDATE seller
SET
trade_name = '".$trade_name."',
s_address = '".$s_address."',
s_image = '".$filename."'
WHERE seller_id= '" . $_SESSION['seller_id'] . "' ");
if ($sql2)
{
header('location: seller_menu.php');
}
else
{
echo "error occured : " . mysqli_error($db);
}
}
else
{
echo "error occured : " . mysqli_error($db);
}
}
?>

Answer 1

The function mysqli_insert_id returns the id of the row you just inserted into your database, and since you don't insert anything (you just update) the value the function returns is 0, so the name of your image is $id.'.'.$extension ==> 0.png.
Since you update the seller_id, and you have it inside $_SESSION['seller_id'], you can use it in your code:
$filename = $_SESSION['seller_id'].'.'.$extension;