set From address in sendmail.php

Hi
What code needs to be in sendmail.php document or elsewhere to include a From address in forms being send from the site?
I can’t see there’s such a definition in my current sendmail.php

<?
$to = "[email protected]";
$subject = "CourseLead";
$content = $_POST['name'] . ", " . $_POST['phone'] . ", " . $_POST['email'] . ", " . $_POST['course'];
$headers ="Content-type: text/plain; charset=utf-8\r\n";

Thank you