Here's a simple example:
<?php
	//add From: header
	$headers = "From: jm3-hide@address.com\n";

	//specify MIME version 1.0
	$headers .= "MIME-Version: 1.0\n";

	//unique boundary
	$boundary = uniqid("HTMLDEMO");

	//tell e-mail client this e-mail contains//alternate versions
	$headers .= "Content-Type: multipart/alternative; boundary = $boundary\n\n";

	//message to people with clients who don't understand MIME
	$headers .= "This is a MIME encoded message.\n\n";

	//plain text version of message
	$headers .= "--$boundary\n" .
	"Content-Type: text/plain; charset=ISO-8859-1\n" .
	"Content-Transfer-Encoding: base64\n\n";
	$headers .= chunk_split(base64_encode("This is the plain text version!"));

	//HTML version of message
	$headers .= "--$boundary\n" .
	"Content-Type: text/html; charset=ISO-8859-1\n" .
	"Content-Transfer-Encoding: base64\n\n";
	$headers .= chunk_split(base64_encode("This the <b>HTML</b> version!"));

	//send message
	mail("hide@address.com", "An HTML Message", "", $headers);
?>
done!