I see curly braces in HTML code to be used to separate the logic (PHP) from the view (HTML) but I can't understand it..
for example in phpBB:
<h3>{L_BIRTHDAYS}</h3>
any help will be appreciated
What you are looking at is parsing code. the php parser will consume the html and when it comes across a pattern, in this case {} it will look in its property bag and substitute the values for the variable names inside the {}
You can read more about this sort of thing by googling rendering engines.