PHP Question

I have a php script that after filling out a form will email a link to a friend:

<?
include "PHP_Lib.php";
$subject = $HTTP_GET_VARS["Subject"];
mail("$Recipient_Email","Please check this link out","$Recipient_First_Name,

Please check this link out: $message

Thanks,
$Contact_First_Name $Contact_Last_Name");
?>

How can I get it so that the "From" address in the email looks like it is coming from the sender?

Thanks!

Jody