It needs some work. The line that you have there for the code isn't setting up a valid attribute, but the bigger problem is its simply missing the /.
'<B>The next step is to click on this link to activate your account: <a href="'.$siteUrl.'/activate.php?username=' .$signup['username'].'">CLICK HERE</a></b>';
Would give you what you want. The markup itself isn't very good, but browsers should understand it.