Send email with a template using php

How can I send an email using php then add a template design in the email? I'm using this:

$to = "someone@example.com";  
$subject = "Test mail";  
$message = "Hello! This is a simple email message.";  
$from = "someonelse@example.com";  
$headers = "From: $from";  
mail($to,$subject,$message,$headers);  
echo "Mail Sent.";  

And it works fine! The problem is just how to add a template.