title: parse_signed_request - base64_url_decode - strstr use
body: <p>in the facebook php reference there the 
  parse_signed_request to check out a signed_request
  I don't understand the use of
  <code>strtr($input, '-_', '+/')</code> in</p>
  
  <pre><code>function base64_url_decode($input) {
     return base64_decode(strtr($input, '-_', '+/'));
  }
  </code></pre>
  
  <p>I've echoed the <code>$input</code> string but at least
  in my case there is no '<code>-_</code>' to replace with '<code>+/</code>'
  Can you explain what's it for ?
  Bye.</p>