<?
if ("mail" == $_GET['act']) {
    mail("admin@example.org","User feedback: ".$_POST['subject'],$_POST['text'],"From: robot@example.org\r\nContent-type: text/plain; charset=windows-1251");
    echo "Послали письмо.";
}
?>
<form action="<?=$_SERVER['PHP_SELF']?>?act=mail" method="POST">
Тема письма: <input type="text" name="subject"><BR>
<textarea name="text"></textarea>
<input type="submit">