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Passing a dynamic variable as a parameter to a form validation callback function

Perhaps this is obvious, but it took me a while to get right so I'll post it here in case it's helpful for anyone.

Normally a CodeIgniter form validation callback function takes one parameter, which is passed to it "automatically": the value of that particular form field.

For example:
function _my_callback_function($field_value){
// Do something to check $field_value, then return TRUE or FALSE

With the corresponding validation rule:
$this->form_validation->set_rules('some_field', 'Some Field Name', 'callback__my_callback_function');

If you want to pass a second parameter (or maybe you'd call it a second argument) to your callback function, you can do it like this:

Callback function:
function _my_callback_function($field_value, $second_parameter){
// Do something to check $field_value and $second_parameter, then return TRUE or FALSE

Validation rule:
$second_parameter = 'foo';
$this->form_validation->set_rules('some_field', 'Some Field Name',
'callback__my_callback_function[' . $second_parameter . ']');

The thing that wasn't obvious (to me) was to put the parameter in square brackets in the validation rule.

Note I've given my callback function a name that begins with an _underscore, so the validation rule contains a __double underscore. And note that if you're doing this and you want to set a validation message for your callback rule, don't forget the underscore there, too:
$this->form_validation->set_message('_my_callback_function', 'My message');

Now you mention it, it WAS obvious.. Thanks a lot !

Supposing your using the config/from_validation.php to set up an array of validation rules. Is it then possible to pass in the value of another field, so the value of this field can be used as part of a db query in the validation?

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