Update parts of interpreter, that render some (x)HTML

Hi.

This pull-request introduces changes on source code, that renders some (x)HTML to end-user. Affected are:

 - `phpinfo()` core function
 - PHP core error messages
 - tests

I have changed very old XHTML code used by actual interpreter according to [HTML — Living Standard specification](https://html.spec.whatwg.org/multipage/).
Please, don't mix responsibilities. Focus on PR's goal.