As orrd101 said, there is a bug with //IGNORE in recent PHP versions (we use 5.6.5) where we couldn't convert some strings (i.e. "∙" from UTF8 to CP1251 with //IGNORE).
But we have found a workaround and now we use both //TRANSLIT and //IGNORE flags:
$text="∙";
iconv("UTF8", "CP1251//TRANSLIT//IGNORE", $text);