If you are having a nich content, there are times you wanted to only let visitor search a few specific keywords, to boost the site keyword ranking or whatever reason. In default, WordPress come with a simple search form, with an input and a button. A lot of users find it quite useless.

While searching for some solution on my project, I find some trick on limiting the keyword, but using a dropdown selection menu.

<select name="s">
<option value="A">A</option>
<option value="B">B</option>
<option value="C">C</option>
</select>

WordPress engine use the name="s" as its listener for keyword. Now that I am using manual selection, I can bascially place only keywords I want the visitor to search for. Well, unless the visitor is a WordPress user too, as there is always possible to manully type in /?s=keyword.