evaluating vars in a var

Hi, this problem came forth from my previously posted -Mail(): how much time-, but is different so I posted it seperately. I make html emails with personal stuff in it e.g. <span class='someclass'>hello $firstName $lastName</span> etc. Problem: I would like to put the whole email html layout in one var, like this: $htmlEmail = "<html><body><span class='someclass'>hello $firstName $lastName</span></body></html>"; and then I want to have the values of firstName and $lastName updated in every iteration of a loop where I put in new Values of these vars. In order to do this, I have to reevaluate the whole $htmlEmail variable. Is this possible, and if so, how? thanks a lot