This works:

$  redirectURL = '/cart/add/p2'; return new RedirectResponse(base_path().$  redirectURL); 

The problem is, I want this:

$  redirectURL = '/cart/add/p2?destination=somewhere'; 

The moment I added destination=somewhere, I get this error:

InvalidArgumentException: The internal path component 'http://localhost/XXXX/public_html/XXXX/XXXX' is invalid. Its path component must have a leading slash, e.g. internal:/foo. in Drupal\Core\Url::fromInternalUri() (line 404 of C:\ampps\www\XXXX\public_html\core\lib\Drupal\Core\Url.php). 

However, if I do this:

$  redirectURL = '/cart/add/p2?SOMETHING_ELSE=somewhere'; 

Then it works. In other words, it specifically breaks if I add “destination=somewhere”.

Why would it be breaking on this and how do I get around it? I.e. How do I redirect to a URL with destination=somewhere in the query string.

FYI: If I can get this to work, I can use “Cart Links” to add stuff to a cart and redirect the user at the same time, making my live a lot easier.