email address from form submission to appear in "From"

How do I code my php script to have the users e-mail address populate the "sent from" in the e-mail message. When the admin responds to the message by hitting "reply", it should go back to the user. Here is the line as it is now, which just puts a static "The web site" in the "sent from":

mail($to_addr, $_REQUEST['subject'].' '.$_REQUEST['name'],$mail_body,'From: The web site>' . "\r\n");

I'm also attaching the entire script. THANKS!