Thinking about it, this is an interesting task. "No database" at first sounded a bit awkward, but thinking about it, it can be done without a database just fine.
How? Google. Google searches are very powerful things. If I wanted to search for a selection of keywords across selected, I'd do it using a google search with a query like
good programming practises site:sitepoint.com/forums/showthread.php OR site:forums.digitalpoint.com/showthread.php
Although the latter domain isn't going to be much use for that, it's an example.
Well, you can also use google from within PHP. It's against their T&Cs to use scraping of Google, of course, but they have an API. https://developers.google.com/custom-search is where you want to check out.