I'm reviewing code of your class at the moment and i'm wondering with that line:
$headers = 'X-Mailer: PHP/' . phpversion();
Why did you write this phpversion() function in here? Is it necessary? To be honest i do not see point of sending server's php version info in header of an email. Or i misunderstand that line?
Overall very nicely written class and example of its use, it is ready to add to any website as it is already.