In the case you're asking an API which gives you a reusable application key, you'll have to use the "verifier" in the getAccessToken() method, but also the "callback" parameter in the getRequestToken().
$request_token_info = $oauth->getRequestToken($req_url, "<YOU_CALLBACK_URI>");
In my case, once I accepted the form in the Authorize URI, I was redirected to this callback URI with a "oauth_verifier" GET parameter. I injected it into the access token request :
$access_token_info = $oauth->getAccessToken($acc_url, '', $_GET['oauth_verifier']);
I finally obtained an access token that I hardcoded ; it doesn't need any refresh.