Major milestone for PHP Community: PHP 5.5 has been released:

http://php.net/archive/2013.php#id2013-06-20-1

So far, XOOPS 2.5.6 runs on PHP 5.5.0 just fine

The only issue so far are with couple of deprecated features:

1) "preg_replace() with the /e modifier"

there are only two files where this is used:

/class/module.textsanitizer.php
/class/mail/phpmailer/class.phpmailer.php

It should be replaced with preg_replace_callback() function

2) mysql extension is deprecated,

In XOOPS 2.6.0 the plan is to switch to mysqli or to PDO, so this will take care of it.

And since both of these features are only deprecated, but still existing, XOOPS should work on PHP 5.5.0

However, if you find anything else, please let us know....