I am learning PHP from from Murach's book and I am facing some issues to understand this code,

```
<nav>
  <ul>
     <?php foreach ($categories as $category) : ?>
     <li>
        <a href="?category_id=<?php echo $category['categoryID']; ?>">
                    <?php echo $category['categoryName']; ?>
        </a>
     </li>
     <?php endforeach; ?>
  </ul>
</nav>

```

My problem is with this part,

```
<a href="?category_id=<?php echo $category['categoryID']; ?>">
     <?php echo $category['categoryName']; ?>
</a>

```

All this is trying to do is to display the category list. So each time user clicks this pass the category ID.Why this is written like that ? why the question mark with category\_id (?category\_id)in the beginning ? is that a variable and assigning it with remaining code ? and why is an echo statement for categoryID in anchor tag ? Because in the webpage it displays only the categoryName. I am just beginner so please excuse me if it sounds so stupid.