I had the problem and error described here: http://xenforo.com/community/threads/8wayrun-com-xenporta-portal.26838/page-37#post-369340
On my local install (also php 5.4) I found the same error on the same spot in the XenPorta application. I fiddled around, and did something (in the "lol I've no idea what I'm doing" fashion) and to my surprise it worked. (EDIT: To be clear, server is running linux, the local install is running on windows. Both use nginx, and have the same config for nginx, php-fpm and mysql.)
In Node.php I looked up line 1255 and changed the following:
'selected' => (strval($selectedNodeId) === '0'),
'selected' => ($selectedNodeId),
My apologies if this isn't a bug, and I suggest for other people to NOT change this in their install.
On another server I had a couple applications breaking because PHP handles things different in their newest version. Perhaps (what I've seen with other applications) is something between the lines of if php is lower than version 5.4 do this, else do that. Also my apologies if this all seems a bit unclear and confusing, I've been trying to get things to work all day, and am tired now.
I hope anybody can help me, and many thanks in advance!