title: Show a number to two decimal places
body: <p>What's the correct way to round a PHP string to two decimal places?</p>
  
  <pre><code>$number = "520"; // It's a string from a database
  
  $formatted_number = round_to_2dp($number);
  
  echo $formatted_number;
  </code></pre>
  
  <p>The output should be <code>520.00</code>;</p>
  
  <p>How should the <code>round_to_2dp()</code> function definition be?</p>