MU-Plugin for WordPress to redirect from non-www to www domain

redirect-to-www.php

<?php

function redirect_to_www() {
	if ( ! preg_match( '#^www.#', $_SERVER['HTTP_HOST'] ) ) {
		$https = 443 === $_SERVER['SERVER_PORT'] ? 's' : '';
		header( "Location: http{$https}://www.{$_SERVER['HTTP_HOST']}", true, 301 );
		die();
	}
}
redirect_to_www();