$category_id = null;

if ( $option == "com_content" and $view == "article" ) {

   /* if it's an article page without the catid mentioned in the url */

   $nullDate = $db->Quote( $db->getNullDate() );

   $my_id = $db->Quote( $db->getEscaped( (int)$id ) );

   $jnow  =& JFactory::getDate();

   $now    = $db->Quote( $db->getEscaped( $jnow->toMySQL() ) );

   $query = "SELECT catid "

       . " FROM #__content WHERE id = $my_id AND state = 1"

       . " AND ( publish_up = $nullDate "

       . " OR publish_up <= $now )"

       . " AND ( publish_down = $nullDate "

       . " OR publish_down >= $now  )";

   $db->setQuery( $query, 0, 1 );

   $row = $db->loadObject();

   $category_id = @$row->catid;



  /* Now customise any of the following rules for your use.

   * $category_id will correspond to the category id of the

   * article being displayed (if one is being displayed!)

   */

  if ($category_id == 1) return 55;/* on category 1, return module 55 */

  if ($category_id == 2) return 56;/* on category 2, return module 56 */

  if ($category_id == 3) return 57;/* on category 3, return module 57 */

  return 50; /* default if nothing else matches */

}