How do I add these php tags to the tag?

How do I add these php tags to the tag?

I am modifying a wordpress function. I have the following code:
$output .= $r['link_before'] . '<ul><li class="bbp-forum-info"><a href="'
. esc_url( $permalink ) . '" class="bbp-forum-link">' . $title .
'</a></li>' . $counts . '<li class="bbp-forum-freshness">' . $subfresh .
'</li></ul>' . $r['link_after'];
And I need the ul tag to look like this:
<ul id="bbp-forum-<?php bbp_forum_id(); ?>" <?php bbp_forum_class(); ?>>
However, I don't know how to add those php tags inside of that code. I
have tried it in a number of different ways, but haven't been able to get
it to work. Any advice?