The default model for the search box is AND, with the two terms space-delimited. An example would be to enter into the textbox the literal string
graphics snake. The results you'll get back are individual posts that contain the word
graphics and also contain the word
snake. Proof:
search.php?keywords=graphics+snake&terms=allThe
| (pipe) operator is used to change the logic from AND to OR; for example
graphics | snake would look for posts that contain the word
graphics or the word
snake and return all those results. I believe this is the same thing as choosing the "Search for any terms" radio button.
Use of double quotes (ex.
"graphics snake")
do not cause the search function to look for the string
graphics snake as an individual word/phrase.
phpBB does not support phrase-searching like this.
There are also numerous settings/adjustments in phpBB itself, such as when building the search results (this happens behind the scenes) there are a minimum number of characters and a maximum number of characters that limit the results; those are set to 3 and 14 right now, respectfully. So if you're looking for a short phrase such as
ok that probably won't get you accurate results. There is also a "common word threshold" percentage adjuster that ignores certain words in queries that meet certain criteria; we have this set to 20% right now. For example the word
if would probably match that condition, since it's an incredibly common word used in posts.