// Checks if USERNAME is set in the session
if(isset($_SESSION['USERNAME'])){
// Grabs username that are unique from the lastactive table, where the date_now is greater than the current datetime
 $stmt = $pdo->prepare("SELECT DISTINCT username from lastactive where date_now > now() - INTERVAL 15 MINUTE;");
 $stmt->execute();
 // Prints out Online Users: as an h4 element with the id of who_title
 echo "<h4 id='who_title'>Online Users</h4>\n";
 // Creates ul element
 echo "<ul>\n";
 // Loops through the rows
 while($row = $stmt->fetch()){
    // Display li tags with username in the tags
  echo "<li>" . $row['username'] . "</li>\n";
 }//end while
 echo "</ul>\n";
}