rodrigo at bb2 dot co dot jp wrote that inconv works better than mb_convert_encoding, I find that when converting from uft8 to shift_jis 
$conv_str = mb_convert_encoding($str,$toCS,$fromCS); 
works while
$conv_str = iconv($fromCS,$toCS.'//IGNORE',$str); 
removes tildes from $str.