<?php
if (substr($_SERVER['HTTP_HOST'],0,3) != 'www') {
header('HTTP/1.1 301 Moved Permanently');
header('Location: http://www.'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
}
?>

Issues a check to see if the "www" is included, if not, forces "www" to be appended to the URL in question.

SEO - 301 Cononical Redirect - Adds WWW

redirect

PHP