replying to email adress on the from

[Anonymous]

heres the process.php. i want to recieve the email from the email adress on the form so i could reply to them

<?php
$where_form_is="http://".$_SERVER['SERVER_NAME'].strrev(strstr(strrev($_SERVER['PHP_SELF']),"/"));
mail("harshad@website.com","Reservaion Enquiry","Form data:
Preferred Arrival Date : " . $_POST['field_1'] . " 
Preferred Departure Date: " . $_POST['field_2'] . " 
Room Category: " . $_POST['field_3'] . " 
Adults: " . $_POST['field_4'] . " 
Children: " . $_POST['field_5'] . " 
Please contact me by: " . $_POST['field_6'] . " 
First Name: " . $_POST['field_7'] . " 
Last Name: " . $_POST['field_8'] . " 
Email: " . $_POST['field_9'] . " 
Phone: " . $_POST['field_10'] . " 
");
include("confirm.html");
?>

[Harald Ferber]

Hi, yes this man is right.
It is logical reply directly to the email of the sender of the form and not to the server which sent the form.
How could I change the code?
Thanks in advance for any help!!!!!