<form action="mail.php" method="post">
<input type="text" name="from">
<input type="text" name="to">
<textarea name="content" rows="4" cols="40"></textarea>
</form>

$to  = $_POST['to'] ;
$subject = $_POST['from'];
$message = $_POST['content'];
mail($to, $subject, $message, //$headers);