why is iconv not working as expected?

Hi there,

I'm puzzled by the iconv() call from php. When I test it with cyrillic 
text, in the utf-8 or windows-1251 charsets, or when I try with iranian 
text, the following php call

<?php
	echo @iconv($charset, 'ASCII//TRANSLIT', $texte))
?>

brings me only question marks "???????".

What I don't understand is that lynx parses those same texts very well, 
and it is (I think?) based on the same iconv() library...


What can I do to either tune my system or do the right call to iconv()? 
The aim is to have a nice search engine for SPIP, a GPL'd CMS, so the 
most portable answer will be the best ;)

(I'm not on the php-dev list, so please Cc: me.)

-- Fil