Dynamic HTML Email

Hello, I am wondering how it would be possible to generate an html response email via a PHP script. I have tried and all that has happened is the html source code appears in the message body as plain text. I believe this is happening because the email needs an additional header stating the email needs to be parsed as html not plain text (or both). I am using the mail command. Can someone shed some light on the subject for me??? Thanks so much in advance! Mike Dunlop