title: Checking for null and missing query string parameters in PHP
body: <p>I want to be able to distinguish between existing query string parameters set to null, and missing parameters.  So the parts of the question are:</p>
  
  <ul>
  <li>How do I check if a parameter <em>exists</em> in the query string </li>
  <li>What's the established method for passing a null value in a query string? (e.g. param=null or param=(nothing) )</li>
  </ul>
  
  <p>Thanks</p>