The reason why I want to use this is for a custom 404 page for example where the user gets redirected to an outdated site by Google or Altavista for example, so I can't use their search engine.

The site does not use a database. I was thinking about defining a flat-file with keywords and corresponding URLs, like this:
vbspot-->/projects_vbspot.php
mares-->/about_sebastianmares.php
acd-->/projects_acd.php
donate-->/participate.php
If the user searches for "vbspot acd", the results "/projects_vbspot.php" and "/projects_acd.php" should be displayed.