PHP in HTML mailer...?

Hey all,
I’ve got a form mailer I’m working on but I hit a snag and I’m not sure what to do. Basically, I am sending out the mailer formatted with html… in order to do this, I’m sending:

$mailer = ‘<html>blahblahblah $variablevalue</html>’;

The html renders out fine in the mail, but the $variablevalue isn’t filled… it just appears as is. “$variablevalue”.

Any ideas how to get this to actually fill when I send the email? thanks