Agrupar produtos por categoria.

index.php

<?php
  $pdo = new PDO("mysql:host=localhost;dbname=loja", "root", "");
  
  $sql = "SELECT p.id, p.name, c.name AS categoryName, c.id AS category_id FROM product p INNER JOIN category c ON c.product_id = p.id ORDER BY c.name ASC";
  $results = $pdo->query($sql);
  ?>
  
  <?php 
  $group = '';
  foreach($results as $result) : ?>
  
     <?php if($group !== $result['category_id']) : ?>
  
      <h2><?php echo $result['categoryName']?></h2>
      
      <?php $group = $result['category_id']?>
     <?php endif;?>
     
     <p><?php echo $result['name']?></p>
  
  <?php endforeach;?>