YL is right. I think I got it now.
The page you linked is opened in the same browser-page! (that is what you have to mean when talking about '_self', '_blank' is openng in a new browser tab, etc).
But you want the content of your totally independent PHP file to open like a blog entry inside the blogs header/sidebar/content page, right?
First, this is not
that easy. You have to embed/include the Serendipity framework in your php file output, to fake this. Search for embedding in the docs, like
http://www.s9y.org/55.html .
To start with, I would install the staticpages event plugin. Then you can generate static pages, eg a "/pages/mystaticpage.html" with some static content and even some more options. This link can now be easily linked in your template navbar and opens in your templates content area.
Now such an static page could include a dynamic php file by some html/js magic.