I got a trick. create a function and call it on callback_column parameter..
$crud = new grocery_CRUD();
$crud -> set_table('booking');
$crud->set_relation('room_id','rooms','name');
$crud->callback_column(unique_field_name('room_id'),array($this,'_visitor_details_popup')); verity
function unique_field_name($field_name)
{
return 's'.substr(md5($field_name),0,8); //This s is because is better for a string to begin with a letter and not with a number
}