<p>Order By: 
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?> method="GET">
    <select name="ordering">
      <option value="pk DESC" id="newest">Newest First</option>
      <option value="pk ASC" id="oldest">Oldest First</option>
      <option value="rating DESC" id="highest">Highest Rating</option>
      <option value="rating ASC" id="lowest">Lowest Rating</option>
    </select>
</form>
</p>

<?php
include "common.php";

mysql_connect("HOST", "USER", "PASS") or die(mysql_error());
mysql_select_db("DB") or die(mysql_error());
if(isset($POST['ordering'])) { 
    $sql = "SELECT email, review, rating FROM reviews ORDER BY" . $POST['ordering'];
} else {
     $sql = "SELECT email, review, rating FROM reviews ORDER BY review_id DESC ";
}
 $result = mysql_query($sql)or die(mysql_error());

  echo "<table>";
  echo "<tr><th>User</th><th>&nbsp&nbsp&nbsp</th><th>Review</th><th>&nbsp&nbsp&nbsp</th><th>Rating</th></tr>";

  while($row = mysql_fetch_array($result)){
    $email     = $row['email'];
    $review    = $row['review'];
    $rating    = $row['rating'];

echo "<tr>
  <td>".$email."</td>
  <td></td>
  <td>".$review."</td>
  <td></td>
  <td>".$rating."</td>
  </tr>";
}
?>  

All you need to do is wrap the select in a form that submits to itself, change the option values to partial SQL queries, and finally make a conditional SQL statement based off of the form input.