title: How do I check if a $_GET parameter exists but has no value?
body: <p>I want to check if the <code>app</code> parameter exists in the URL, but has no value. </p>
  
  <p><strong>Example:</strong><br /></p>
  
  <pre><code>my_url.php?app
  </code></pre>
  
  <p>I tried <code>isset()</code> and <code>empty()</code>, but don’t work. I’ve seen it done before and I forgot how. </p>