I guess something like:

$stop = false;
while($stop == false)
{
    if(substr($keywords, 0, 3) == 'or ')
    {
        $keywords = substr($keywords, 3);
    }
    elseif(substr($keywords, 0, 4) == 'and ')
    {
        $keywords = substr($keywords, 4);
    }
    else
    {
        $stop = true;
    }
}

would work, but then I'm not entirely sure what's the purpose of it (what exactly does it prevent?).

and and or are keywords and can't be used as search term. This code is needed because the regex to split the search term doesn't find and and or in the beginning: #\s(and|or)\s#
Your suggestion would not catch a simple and. I'd probably use a regex here, something like:

$keywords = preg_replace('#^(and|or)(\s|$)((and|or)\s)*#', '', $keywords);

Ah yes, forgot about single keyword.

Are those MyBB-specific keywords? What's their function or rather usage (it could be documented somewhere)?
EDIT: nvm, figured it's keyword or keyword and keyword and keyword, but I'm still not sure how's a regular admin/user supposed to know about this functionality.
As for regex, this is slightly simplier:

$keywords = preg_replace('/^((and|or)(\s|$))+/', '', $keywords);

While a regular user doesn't need to know, searching for and or or would be quite useless given to the fact both words are that common. And you can still search for it in a phrase: "foo and bar"

btw: These keywords and search expressions are quite common. Fulltext search supports some more things, too.

While a regular user doesn't need to know

Noone needs to know, but then what's the point of having an unused feature?

I think it wouldn't hurt to add some tips under the keyword textbox: http://community.mybb.com/search.php depending on the search type. I'd do that in the PR I'm just preparing but then I'm not sure of all tricks myself. Could you list them?

It depends whether fulltext search is used or not.
Both search systems support AND, OR, *, and phrases ("foo bar"), fulltext additionally allows NOT, +, -, ~, <, >, @ and parentheses groups (explanations and examples: http://dev.mysql.com/doc/refman/5.6/en/fulltext-boolean.html)

Doesn't regular search also support *?

You are right. I've missed that.

@Destroy666x You seem to have made a commit which seems to fix this but never created a PR with it? Is there anything missing?

@Destroy666x @effone @Shade- and others devs - there is a FIX from Destroy in his repo, can you check if it is fine? I am trying to test it on my board, seems fine but needs proper testing. I can recreate the PR if it will be fine. Let me know, thank you!

@Destroy666x @effone @Shade- and others devs - there is a FIX from Destroy in his repo, can you check if it is fine? I am trying to test it on my board, seems fine but needs proper testing. I can recreate the PR if it will be fine. Let me know, thank you!

It broke SQL request with ' inside, no SQL filter. Maybe there are security issue : tested with Cahiers d'Ulysse

This one is OK #2349 (comment) and really more simple.

I think its the best fix.

Hmm, maybe some discussion?

Hmm, maybe some discussion?

PS : #4532 but still not fixed in feature :

If users can't use these words (and, or) we could add some warning while typing (there is no such feature altogether as of now).

Regardless of ultimate fix regarding the front-end some in-line form verification should be added at some point and consider this specific case if so.

If users can't use these words (and, or) we could add some warning while typing (there is no such feature altogether as of now).

It's not THIS word, it's allw aord styarting by or and and …

orange for example

Fixed by a simple edit #4707

@Destroy666x @effone @Shade- and others devs - there is a FIX from Destroy in his repo, can you check if it is fine? I am trying to test it on my board, seems fine but needs proper testing. I can recreate the PR if it will be fine. Let me know, thank you!

It broke SQL request with ' inside, no SQL filter. Maybe there are security issue : tested with Cahiers d'Ulysse

I can not reproduce with the fix and search "Cahiers d'Ulysse" return Cahiers d'Ulysse topics.