Well, in the olden days, you would be using a Frameset and a target of the content area frame, but these are not the olden days, so things have changed.
PHP is a server-sdie scripting language and it requires the link to be processed back at the server, so that would be out of the question, apparently.
That leaves javascript, or AJAX, which is javascript with http requests and junk, which does not require a full page request, however, it might not be the wisest thing to do here.
Simply refresh the current page with a php request and pass the contents as a query string.
To simplify the issue, have a look at using a Content Management System would be my suggestion.