First of all, welcome to the boards. Make yourself at home. Have a look around, see what there is to see, feed the dog (he's just a big softy, really).
For your problem, (not being a PHPBB user myself), it looks to me you can avoid PHP itself entirely for this issue: this being the idea behind the template system. A wee bit of added HTML in the template should do the trick. Specifically, a line break tag somewhere in that line, 'bout halfway along.
<a href="{U_ALBUM}" class="mainmenu">
change to
<br><a href="{U_ALBUM}" class="mainmenu">
Then you'll get it on two lines.
(Incidentally, I didn't see what you meant until I went down to 800x600. Or maybe you meant something else.)
