HELP!!! Fatal error: Call to undefined function db_num_rows() in public_html/includes/common.inc(1685) : eval()'d code on line 9

I was trying to add a new block to my website. I copied the following code from this site: http://drupal.org/node/148911 and pasted it into a new php block.

As soon as I enabled the new block, my page went blank and i got the following error!
Fatal error: Call to undefined function db_num_rows() in public_html/includes/common.inc(1685) : eval()'d code on line 9

I can't get back into my admin to delete the block, every page I try to go to, I get that message! Please tell me this is something I can fix!

<?php
// Count users with activity in the past defined period.
$interval = time() - 900;

// Perform database queries to gather online user lists.  We use s.timestamp
// rather than u.access because it is much faster is much faster..
$anonymous_count = sess_count($interval);
$authenticated_users = db_query('SELECT u.uid, u.name FROM {users} u INNER JOIN {sessions} s ON u.uid = s.uid WHERE s.timestamp >= %d AND s.uid > 0 ORDER BY s.timestamp DESC', $interval);
$authenticated_count = db_num_rows($authenticated_users);

// Format the output with proper grammar.
if ($anonymous_count == 1 && $authenticated_count == 1) {
  $output = t('There is currently %members and %visitors online.', array('%members' => format_plural($authenticated_count, '1 user', '@count users'), '%visitors' => format_plural($anonymous_count, '1 guest', '@count guests')));
}
else {
  $output = t('There are currently %members and %visitors online.', array('%members' => format_plural($authenticated_count, '1 user', '@count users'), '%visitors' => format_plural($anonymous_count, '1 guest', '@count guests')));
}

echo $output;
?>