Passing Querystring Variable to PHP Code in Plugin
If you have a URL like this:

mysite.com?username=jsmith


You can retrieve it from your plugin via craft()-&gt;request-&gt;getQuery('username');

See here: https://craftcms.com/classreference/services/HttpRequestService#getQuery-detail


  My second question is - can Craft (Twig template) pass to the PHP plugin by including the PHP file (service)?


I'm not actually following this question, but we generally try to keep to one question per post.  I'd suggest posting a new one for that with some additional details of exactly what you're trying to do.

Would also suggest giving the official plugin documentation a read-though: https://craftcms.com/docs/plugins/introduction