Updating multiple database rows using one form

I am trying to bulk update entries in a table using one form. I have seen other examples of this but I cannot seem to get them to work. I do not get any errors when I submit the form, but it will not update my db. I have experimented with a variety of methods. All I want it to be able to use a checkbox to select the entries that are approved, and then submit it to the db. Any advice is appreciated.

<form action="early_reg.php" method="post">
  <table align="left">
    <tr>
      <td align="left">Early Registration</td>
    </tr>
  </table>
  <br />
  <br />
  <table border='1' align='left' bordercolor='#000000' cellpadding='0' cellspacing='0'>
    <?php

  //QUERY ENTRY
  $sql = ('SELECT * FROM S7preregistration order by pkPreregID ASC');
  $result = mysql_query($sql);
  $num_rows = mysql_num_rows($result);

  // ENTRY EXISTS
  if (!($num_rows == 0)) {

    $myrow = mysql_fetch_array($result);
    do {
      printf ('<tr>');
      echo "<td><input type='checkbox' name='approved[]' value='yes' /></td>";
      printf ('<td align="left">%s</td>', $myrow['teamName']);
      printf ('<td align="left">%s %s</td>', $myrow['firstName'], $myrow['lastName']);
      printf ('<td align="left">%s</td>', $myrow['emailAddress']);
      printf ('<td align="left">%s</td>', $myrow['phone']);
      printf ('<td align="left">%s</td>', $myrow['description']);
      printf ('<td align="left">%s</td>
      </tr>', $myrow['signupDate']);
    }
    while ($myrow = mysql_fetch_array($result));
  } 

 echo "<tr>
        <td colspan='7' align='left'><input type='submit' name='update' value='UPDATE' /></td>
    </tr>";

//SUBMIT THE INFO
if(isset($_POST['update'])){
    foreach($_POST['approved'] as $approved){
    mysql_query("UPDATE S7preregistration SET approved='$approved' WHERE pkPreregID='$pkPreregID'");   
    }
}
?>
  </table>
</form>