title: I click on a category, but the filter based on title of the category, is not working
body: <p>I have a page wherein my sidebar, I list all the categories that I have in my database, it is showing as in the image,</p>
  
  <p><a href="https://i.stack.imgur.com/yDYBN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yDYBN.png" alt="enter image description here"></a></p>
  
  <p><br></p>
  
  <p>It is showing based on my database, as shown below,</p>
  
  <p><a href="https://i.stack.imgur.com/wRo62.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wRo62.png" alt="enter image description here"></a></p>
  
  <p><br></p>
  
  <p>And this is my code for the category,</p>
  
  <pre><code>&lt;?php
  include('db.php');
  
     $r = mysqli_query($db_connect, "SELECT * FROM category order by title asc");
  
     if($r) {
        while($row = mysqli_fetch_array($r)) {
           echo '&lt;a href="category.php?filter='.$row['title'].'" class="list-group-item"&gt;'.$row['title'].'&lt;/a&gt;';
        }
     }
  ?&gt; 
  </code></pre>
  
  <p><br></p>
  
  <p>The problem is, why when I click at the category, it is showing me an error page? </p>
  
  <p><a href="https://i.stack.imgur.com/P3SK0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/P3SK0.png" alt="enter image description here"></a></p>
  
  <p><br></p>
  
  <p>Do I have to include any other code or else? Help me please, thank you. <br>
  <strong>I KNOW MY CODES ARE VULNERABLE, JUST IGNORE IT</strong></p>