PHP defining undefined classes in MVC directories - php

Question

So I have four classes in a model directory but my index.php file for a web application can not access the classes. The IDE keeps telling me my that my classes are undefined and when I run the web-app I get these errors:
Notice: Undefined variable: CategoryDB in C:\xampp\htdocs\ex_starts\ch14_ex1\product_manager\index.php on line 28
Fatal error: Call to a member function getCategory() on a non-object in C:\xampp\htdocs\ex_starts\ch14_ex1\product_manager\index.php on line 28
I thought using require statements would include all the classes and their functions but I seem to have made a mistake. Could someone point out where I went wrong?
Here is the index.php require statements:
<?php
require('../model/database.php');
require('../model/category.php');
require('../model/category_db.php');
require('../model/product.php');
require('../model/product_db.php');
$CategoryDB = new CategoryDB();
$ProductDB = new ProductDB();
$action = (if and else statements with class method calls below) {
if ($action == 'list_products') {
$category_id = filter_input(INPUT_GET, 'category_id',
FILTER_VALIDATE_INT);
if ($category_id == NULL || $category_id == FALSE) {
$category_id = 1;
}
// Get data (the CategoryDB and ProductDB have an undefined class warning here)
$current_category = $CategoryDB->getCategory($category_id);
$categories = $CategoryDB->getCategories();
$products = $ProductDB->getProductsByCategory($category_id);
else if{
//code here
}
}
?>
Class names are correct but every function call gives the same class undefined warning in my IDE. Any help in understanding what the correct way to do this would help and be appreciated, Thanks!