<form action="index.php" method="POST">
<input name="numcas">
<input type="submit" onclick="send()" value="Отправить на почту">
</form>
<?
$message = $_POST['numcas'];
$to = "...@bk.ru";
$from = "center.org";
?>
<script>
function send(){
<?mail($to, $pagetitle, $message, "Content-type: text/plain; charset=\"utf-8\"\n From: $to");?>
}
</script>