title: nvl() function in php file
body: <p>I'm just looking through a php file and have come across the following code:</p>
  
  <pre><code>switch (nvl($mode))
  {
    case "add" :
       print_add_category_form(nvl($id, 0));
       break;
  
    case "edit" :
       print_edit_category_form($id);
       break;
  }
  </code></pre>
  
  <p>What does the nvl() function do??</p>