I have a dropdown menu like this:
You can go ahead with sometimes validation for laravel. you can define a custom closure as in the below example
public static myfunction(){
$input = \Input::only('selection','stext');
$rule = array(
'selection' => 'required'
);
$validator->sometimes('stext', 'required', function($input){
return (($input->selection == 1) || ($input->selection == 2));
});
$validate = \Validator::make($input,$rule);
}