Assuming you didn't type it wrong on accident, it appears there is a single quote missing:
Current Code:
$query = "insert into ".$this->catagory."
values ('".NULL." , '".$this->filesRead[$k]."')";
New Code:
$query = "insert into ".$this->catagory."
values ('".NULL."' , '".$this->filesRead[$k]."')";
Notice the single quote after the NULL? 🙂 If you could also include the error you are getting in case this isn't the problem (or simply a typo).
John Cornett