Can I convert empty strings to NULL with a Codeigniter validation rule?

I did it and it’s working fine, with strings, and also with numeric values. 
Here is my /application/core/MY_Input.php

PHP Code:

class MY_Input extends CI_Input {
    public function post($index = NULL, $xss_clean = NULL)
    {
        $value= $this->_fetch_from_array($_POST, $index, $xss_clean);
        return $value === '' ? null : $value;
        //return isset($value) ? $value : (isset($default_fail) ? $default_fail : NULL);
    }
} 


One thing to be aware of is your code introduces a bug when you ask CI_Input::post() to return the entire $_POST array. i.e.

PHP Code:

$data = $this->input->post(); 


That might not be something you ever do, but future code maintenance or changes might run up against this. It's an easy thing to forget and a hard thing to point out to someone else coming to the project.

I think you have to either override _fetch_from_array() - which is not as trivial as what you've done - or,  you could do the following

PHP Code:

public function post($index = NULL, $xss_clean = NULL)
{
    $data = $this->_fetch_from_array($_POST, $index, $xss_clean);
    if(is_array($data))
    {
        $output = array();
        foreach($data as $key => $value)
        {
            $output[$key] = $value === '' ? NULL : $value;
        }
        return $output;
    }
    return $data === '' ? null : $data;
} 


Have you considered that you might want to trim the input in case somebody enters a string with empty spaces?