Check the code below. It works fine.
header('content-type: text/html; charset=utf-8'); function strtolower_rus($string) { $upper = array('А','Б','В','Г','Д','Е','Ё','Ж','З','И','Й','К','Л','М','Н','О','П','Р','С','Т','У','Ф','Х','Ц','Ч','Ш','Щ','Ъ','Ы','Ь','Э','Ю','Я',' '); $lower = array('а','б','в','г','д','е','ё','ж','з','и','й','к','л','м','н','о','п','р','с','т','у','ф','х','ц','ч','ш','щ','ъ','ы','ь','э','ю','я',' '); return str_replace($upper, $lower, $string); }
$result = strtolower_rus('БНAKTARAZ BHATTA');
echo $result;