You could even use something like a skeleton index.html file like this:
/var/www/html/skeleton/index.html:
<body>
Welcome to %USERNAME%'s new page
</body>
and then use the following PHP to update and copy the file into the new directory.
$page = str_replace('%USERNAME%',$username,implode('\n',file('/var/www/html/skeleton/index.html')));
$fh = fopen("'/var/www/html/$username/index.html","w+");
fputs($fh,$page);
fclose($fh);
The first line looks complicated but it's actually quite simple. It loads the skeleton file into an array (using the file() function) and then turns it into a long string seperated by new line's (the \n's) using the implode() function and then replaces occurrences of %USERNAME% with the contents of the $username variable.
Cheers,