[Solved] Delete row if key code is there for more than 15 min

Just like the first "where" clause, add two extra paramaters NULL and FALSE to it:

PHP Code:

    $this->db->where('date_modified < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 15 MINUTE))',NULL,FALSE); 


This will override the normal syntax of db->where, which takes 2 parameters: fieldname and conditional value.