title: what do the 'q' and 'p1', 'p2' mean in URL
body: <p>I am learning how to construct a website. In one demo codes for a demo website, one url is as this: </p>
  
  <pre><code>http://localhost/test.php?q=_&amp;p1=_&amp;p2=_
  </code></pre>
  
  <p>I know that this URL will run <code>test.php</code> in the <code>/var/www/html/</code>. But what do <code>?</code>, <code>q=_</code>, <code>p1=_</code>, <code>p2=_</code> and also the underscore <code>_</code> mean? </p>