为什么需要单点斜杠来表示php中的当前目录

I needed to do use "one dot before the slash" to make this PHP header redirect instruction work:

header("Location: ./retrieve.php?hash=$hash");

I understand that ./ means current directory.

So then why didn't plain header("Location: retrieve.php?hash=$hash");, without the "single dot slash," work?

Is it due to the query fragment ?hash=$hash ?

Update: In a different project, I see that I did this: header('Location:page2.html'); and it worked. Any thoughts on why I may not have had to precede with ./ there?