phpBB is just fine. But the search results depend on the search engine used. By default, phpBB uses its native search engine, which by default indexes words with a minimum of three characters. This can be changed to e.g. two. However, if phpBB is set to use another search backend, the settings are different and possibly more limited. For instance, if the MySQL search engine is set as backend, words with less than four characters are ignored and this cannot be changed.
On the SharpBASIC forum I tested the phpBB native search engine with minimum characters set to two:
10 of the 11 search results are still irrelevant because results with va and first are also shown (underscore interpreted as space). But the most relevant result va_first is shown at the top.Search found 11 matches: va_first
Searched query: va first