You concatenate strings with the period operator in php. i.e.
Code:
$filelist .= "<a href=\"" . $file . "\">" . $file . "</a>";
php is also capable of parsing unambiguous variables within double-quoted strings, so this works as well:
Code:
$filelist .= "<a href=\"$file\">$file</a>";
Personally, though, I avoid the latter method for clarity's sake.