title: make sure atleast one checkbox is selected php
body: <p>I have a little mailing form with a few checkboxes. At least one of the boxes need to be selected before mailing should start.</p>
  
  <p>My HTML:</p>
  
  <pre><code>&lt;input type='checkbox' id='part1' name='box1' value='box1' checked&gt;
  &lt;label for="part1"&gt;Voor Leden agenda&lt;/label&gt;
  &lt;br/&gt;
  &lt;input type='checkbox' id='part2' name='box2' value='box2' checked&gt;
  &lt;label for="part2"&gt;Voor Leiding agenda&lt;/label&gt;
  &lt;br/&gt;
  &lt;input type='checkbox' id='part3' name='box3' value='box3' checked&gt;
  &lt;label for="part3"&gt;Verhuur agenda&lt;/label&gt;
  &lt;br/&gt;
  &lt;button type='submit' name='send'/&gt;send&lt;/button&gt;
  </code></pre>
  
  <p>My PHP:</p>
  
  <pre><code>if (isset($_POST['box1'])) {
      $box1 = 'yes';
  } else {
      $box1 = 'No';
  }
  if (isset($_POST['box2'])) {
      $box2 = 'yes';
  } else {
      $box2 = 'No';
  }
  if (isset($_POST['box3'])) {
      $box3 = 'yes';
  } else {
      $box3 = 'No';
  }
  </code></pre>
  
  <p>i would like to have a script that gives a message like below if no checkbox is selected:</p>
  
  <pre><code>if()
  {
      echo "&lt;p class='redfont'&gt;no checkboxes are selected&lt;/p&gt;";
      echo "&lt;p&gt;&lt;a href='javascript:history.back();'&gt;Click to go back&lt;/a&gt;&lt;/p&gt;";
  }
  </code></pre>
  
  <p><strong>edit:</strong>  how can I give this message with php, only if all boxes are unchecked</p>