I need help with the contact form, only some characters come to the mail

Нужно указать кодировку при отправке данных.

$to = "user@mail.ru";
$from = "admin@example.com";
$subject = "=?utf-8?B?".base64_encode("Тема сообщения")."?=";
$headers = "From: $from\r\nReply-to: $from\r\nContent-type: text/plain; charset=utf-8\r\n";
mail($to, $subject, $message, $headers);

PHP

$to = "user@mail.ru";

$from = "admin@example.com";

$subject = "=?utf-8?B?".base64_encode("Тема сообщения")."?=";

$headers = "From: $from\r\nReply-to: $from\r\nContent-type: text/plain; charset=utf-8\r\n";

mail($to, $subject, $message, $headers);