PHPBB... confusing code!

PHPBB forums reference code by using {}. I am guessing the pages include a file with all of these "variables?".

I don't know, its so confusing, here:

<!-- IF S_DISPLAY_SEARCH and not S_IN_SEARCH -->
<div id="search-box">
<form action="{U_SEARCH}" method="post" id="search">
<fieldset>
<input name="keywords" id="keywords" type="text" maxlength="128" title="{L_SEARCH_KEYWORDS}" class="inputbox search" value="<!-- IF SEARCH_WORDS-->{SEARCH_WORDS}<!-- ELSE -->{L_SEARCH_MINI}<!-- ENDIF -->" onclick="if(this.value=='{LA_SEARCH_MINI}')this.value='';" onblur="if(this.value=='')this.value='{LA_SEARCH_MINI}';" />
<input class="button2" value="{L_SEARCH}" type="submit" /><br />
<a href="{U_SEARCH}" title="{L_SEARCH_ADV_EXPLAIN}">{L_SEARCH_ADV}</a> {S_SEARCH_HIDDEN_FIELDS}
</fieldset>
</form>
</div>
<!-- ENDIF -->


Where does "<!-- IF S_DISPLAY_SEARCH and not S_IN_SEARCH -->" get the value from to see if it is true/false?

<a href="{U_SEARCH}" title="{L_SEARCH_ADV_EXPLAIN}">{L_SEARCH_ADV}</a>
Where does the data for {U_SEARCH} come from? I want to do soemthing simular to this so that I can shorten my code, and so that I can you the
<!-- IF !P_EDITABLE --> on my pages instead of typing:

<?php if (P_EDITABLE) == false) {
code
}
?>

Eh, I am so confused to as how this code works...

these are likely constants that are defined some where in the forum software.
your best bet would be to ask this question on the PHPBB support forums: http://www.phpbb.com/community/viewforum.php?f=46

PHPBB uses a proprietary template parser.