title: Visitor using URL that doesn't exist
body: <p>I had someone visit my site today from a link like this: </p>
  
  <pre><code>www.example.com/pagename.php?_sm_byp=iVVVMsFFLsqWsDL4
  </code></pre>
  
  <p>Can someone explain to me how that works since my actual URL ends with <code>pagename.php</code> and I never allowed a user to input any PHP query and never have session IDs or anything similar.</p>