Prepping '0' value to NULL - php

Question

In CodeIgniter, I want to prep a value returned from a form so that if it is a 0, it will actually be inserted as NULL.
I created a function outside of my controller class:
function prep_zero_to_null($int)
{
if ($int == 0)
{
return NULL;
}
else
{
return $int;
}
}
And at the form validation, I do:
$this->form_validation->set_rules('category_id', 'Category',
'required|integer|prep_zero_to_null');
However, CI still tries to insert zeroes as '0' in the database, which breaks one of my foreign key constraints.
Interestingly enough, if I replace NULL by, say, 25 in the prep_zero_to_null function, CI will indeed recognize it and insert 25 instead of '0'. So my prepping function is indeed getting called, but CI won't allow NULL as a result of it and instead converts it to '0'.
How can I achieve what I want?
Edit: For those wondering, the category_id field does allow null:
`category_id` int(10) unsigned DEFAULT NULL
And the exact error is:
INSERT INTO `articles` (`category_id`, `order`, `title`, `text`)
VALUES ('0', '0', 'test', 'test')
^
Should be NULL
Cannot add or update a child row: a foreign key constraint fails
(`db`.`articles`, CONSTRAINT `articles_ibfk_1` FOREIGN KEY
(`category_id`) REFERENCES `categories` (`id`)
ON DELETE SET NULL ON UPDATE CASCADE)

Answer 1

Just looking at this quickly I think the problem is your $int == 0. Is $int an actual 0 type integer or is it a string? In which case the proper check would be $int == '0'.

Answer 2

Codeigniter validation functions doesn't set the field value based on what you return, your validation function should either return TRUE or FALSE to state that something is valid or not.
If you're after changing the value of something, you'll need to accept variable by reference and modify it in the function then you can return TRUE so it passes the validation.
The best solution would be to make the check before inserting the data into database & not relying on the validation library to do this kind of dirty work.

Answer 3

If you want to insert null to the database you need to return a string with the value "null".
function prep_zero_to_null($int) {
return ($int == 0) ? 'NULL' : $int;
}

Answer 4

Have you tried just unsetting the variable. It's a bit ugly, but it should return NULL for the value then.
Tested here.

Answer 5

I submitted an issue at the GitHub bug tracker for CodeIgniter, as this appears to be a bug.
https://github.com/EllisLab/CodeIgniter/issues/2563
Right now, the workaround at model level is the following:
$category_id = prep_zero_to_null($this->input->post('category_id'));
$data = array
(
'category_id' => $category_id,
'order' => $this->input->post('order'),
'title' => $this->input->post('title'),
'text' => $this->input->post('text')
);
Edit: Apparently, this is the right approach, as there should only be strings at validation/controller level.