OAuth2 access token request invalid - grant_type expecting \"authorization_code"\

The "grant_type" parameter value should literally be the string "authorization_code", and only the "code" parameter value should be the authorization code string you received. 

That is to say, your parameter line should like something like this instead:

    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(array('code'=> $_GET['code'], 'client_id' => "5n5**********aw", 'client_secret' => "zq**********tn8", 'grant_type' => "authorization_code", 'redirect_uri' => 'http://localhost/optiSurface-dropbox/login.php')));

By the way, you linked to the old version of the documentation. For reference, the current version is here: https://www.dropbox.com/developers/documentation/http/documentation#oauth2-token 

Problem solved!! 

I just realized it's not necessary to include the complete path from the URL. Since the app has permission to access just it's own folder, I just included the file path itself.

So my json file it's like this:

{
    "path": "/text.txt",
    "include_media_info": false,
    "include_deleted": false,
    "include_has_explicit_shared_members": false
}

Thank you!!