PHP - Null Value At End Of For() Loop

Hi, I am trying to expand my very limited PHP by building a CMS. It's all new to me so please be courteous with your replies ?

I little history of my problem.

I am running xampp on a windows 10 running php 5.6.38

I have taken two video courses and on each I got stumped halfway through with a very similar problem.
The first was using mysql which I was converting to mysqli until I run into a problem.
I then started a new course which used mysqli but it was from 2014 so not sure which version of PHP or if it would even matter.

I am copying everything word for word and even downloaded the course file on the first one and still got the same problem so I'm wondering if there is something wrong with my setup - config?.

The problem.
I am trying to pull data from the database using a while loop. Each time it returns NULL.

This is the code:
 

<?php 

error_reporting(E_ALL);
ini_set('display_errors',1);

include("includes/database.php");
include("includes/header.php");

if ($connection) {        
    echo "Great, we're connected to the database<br><br>";
    }else {
        echo "Bummer, it didn't connect to the database!";
        }

     $query = "SELECT * FROM categories";
   $result_set = mysqli_query($connection, $query) ;

   if (!$result_set) {
           die("Query Failed".msqli_error($connection));
               }

   while ($row = mysqli_fetch_array($result_set)) {
               echo $row['cat_title'];
               }

    var_dump($result_set);                      
        
 ?>

This is the response:

Great, we're connected to the database

object(mysqli_result)#3 (5) { ["current_field"]=> int(0) ["field_count"]=> int(2) ["lengths"]=> NULL ["num_rows"]=> int(0) ["type"]=> int(0) }

Question:
the database table (categories) is populated with two rows (id & cat_title) and there is data in there.
So why does it not retrieve the data?

Thanks for your help.

$firstArray = array(
'row1' => array(
'dates' => array(
'2014-01-01' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-01-02' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-01-03' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-01-04' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-01-05' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-01-06' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-01-07' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
)
),
'row2' => array(
'dates' => array(
'2014-02-01' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-02' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-03' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-04' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-05' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-06' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-07' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-08' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
'2014-02-09' => array('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3', 'key4' => 'value4', 'key5' => 'value5', 'key6' => 'value6', 'key7' => 'value7', 'key8' => 'value8', 'key9' => 'value9', 'key10' => 'value10'),
)
)
);

$placeDataHere = array();
foreach($firstArray as $key => $dates) {
foreach($dates as $date => $values) {
foreach($values as $key => $value) {
$placeDataHere['DV_' . $date]['SM_' . $key] = 'KS_' . $value;
//Followed by another ~50-70 lines of processing the 3 loop's data..
...
...
....
....
....
....
....
....
}
}
}