Your while loop doesn't make much sense now you've removed the PDO 'stuff' from it. A while loop checks for a truthy value. In Your original function:

<?php

    if ($result_resources -> num_rows >= 1) {
      while($row = $result_resources -> fetch_assoc()){

$result_resources is a PDO object. You're looping through each row in the database (dependent on your query) and that row is being assigned to the $row variable as an array. This statement will continue to be true - i.e. the while loop will continue to run - until you've run out of rows. When you've run out of rows, $row will be equal to null - not an array (in php, a non empty array is true and an empty array or null will be false) - and the statement is false. Here endeth the while loop.

In your new code $article_resources is no longer a PDO object. Instead it's an array of a single row returned from the database. Therefore your statement here:

<?php

 while($resource = $article_resources) {

Is just assigning the $article_resources array to the $resource variable over and over again. Remember a non empty array returns true - meaning this statement is always true, meaning your while loop is forever.

Your $article_resources variable at this point only contains data from one row. To get and return all the rows in the article resources variable, you'll need a different method on your pdo object.

<?php

$result = $sth->fetch_assoc(); // this returns one row

// to:

$article_resources = $results->fetchAll(PDO::FETCH_ASSOC); // this returns all the rows

$article_resources should now contain an array of multiple rows etc..

PHP has an inbuilt function to loop around all the items in an array called a forach loop - as already mentioned :-)

<?php

foreach($article_resources as $resource) {
        include(ROOT_PATH . 'includes/partial-resource-item.html.php');
    }

RE your MVC - separation of concerns, in your case here it's still a little confused - but this isn't the end of the world. Your 'Model' is looking okay because it's a file that handles everything to do with your resource - it's an interface that the application can interact with and have information returned. Yay!

Your controller and view are almost one in the same. The view you've described above is indeed a view - more specifically a 'partial'. It's a small piece of reusable display code, so it makes sense to have this separated.

Your controller is the only section where you've not quite separated out your concerns to MVC. I.e. you shouldn't have any code that outputs to the screen in your controller file. Strictly speaking, a controller receives a request, decided what is has to do such as redirecting, sending mail, returning a view - all good things. In your sections of includes, you're including the header (I guess navigation etc...) with function includes. You'll want to try and separate those.

There's a course on Treehouse for Laravel, which is one of the most popular MVC frameworks for php. After that course, you should have a really clear understanding of Models, Views and Controllers.

This is going one step further so you might want to ignore this until you've got it working again. If you haven't got onto binding your data yet, I would almost certainly bind your parameters instead of including them in your query string. PHP Docs for binding params

<?php


  try{
    $results = $db->prepare(
    "SELECT *
    FROM resources
    WHERE type = :type
    ORDER BY recommended DESC";
);
    $results->bindParam(':type', $type);
    $results->execute();
  } catch (Exception $e) {
    echo "Data could not be retrieved from the database.";
    exit;
  }

$resources = $results->fetchAll(PDO::FETCH_ASSOC);