PHP isn't capable of figuring out whether the email client is capable of displaying HTML or not.
I mean think about it. What happens when you send an email to someone? It sends it from your server to another server, and then the recipient downloads it from the server with whatever email client he wishes to use. He might use Outlook, outlook express, or many others. How is PHP to know?
The only thing you can do is ask them what kind of email they want to receive. Text or HTML.
Even if you tried to guess, and did if statements, (if($email=="hotmail") { //send as html } ) you can still use Outlook and probably many other email clients for web based email, and it depends on the client he uses.
