passing a variable to callback function in codeigniter

You must only set your value!

$var1 = 'some conditions';
$this->form_validation->set_rules("callback__is_value_unique[value, $var1]");

public function _is_value_unique($value, $var1){
echo $var1;
die;
}

From the docs..

If you need to receive an extra parameter in your callback function, just add it normally after the function name between square brackets, as in: "callback_foo[bar]", then it will be passed as the second argument of your callback function.

Sounds like you could only pass one extra argument. Which should be a string. If you want to pass more arguments, you could store them somewhere else and just pass an argument, which stores the location of the additional param.

$index = count($this->arguments);
$this->arguments[$index] = array('value', 'some conditions'/*, ...*/);
$this->form_validation->set_rules("callback__is_value_unique[$index]");

public function _is_value_unique($value, $index){
    $args = $this->argumts[$index];
    echo $args[1];
    die;
}