Problem with a mysql query in running a cron

I am trying to run a cron job whereby the expired items on my site will be cleaned up once they've reached their expiration date.  When I go to run the script that will execute this function I keep getting this message

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /home/school/public_html/cron.php on line 1069

Here is the code

$admin_old_items="select * from admin_settings where set_id=20";

$admin_old_res=mysql_query($admin_old_items);

$admin_old_row=mysql_fetch_array($admin_old_res);



$old="select * from placing_item_bid where (todays(now()) - todays(expire_date) ) >= ".$admin_old_row['set_value'];

$old_res=mysql_query($old);

while($old_row=mysql_fetch_array(old_res))

{

$del="delete  from placing_item_bid where item_id=".$old_row[item_id];

$del_sql=mysql_query($del);



$del="delete  from placing_bid_item where item_id=".$old_row[item_id];

$del_sql=mysql_query($del);

}  
 
The line in question on the error message is
while($old_row=mysql_fetch_array(old_res))

Any help in resolving this would be great.