Drupal - Can I use PHP 5.5?

In short, if your site runs on PHP 5.3, it will run on 5.5 as well, go upgrade! (Your dev environment, until a real release is out)

With the short answer out of the way, here are some more details.

With regard to "Does Drupal work on it", very little changed from PHP 5.3, to 5.4 to 5.5. The one issue that showed up under PHP 5.4 was really just PHP being better at telling developers when they were doing something wrong, rather than an incompatibility introduced by PHP. The details of that are well covered in PHP 5.4 and Drupal 7.

No similar new warnings show up with PHP 5.5 though, and while some things have changed, they are highly unlikely to have negative impact on your Drupal site.

One performance concern that was raised with running 5.4 was that APC was lagging behind in support. This will not be an issue this time, as 5.5 now includes a replacement for APC Optimizer+

Drupal 6: By now, most things work well with PHP 5.3, meaning it should work well on 5.5 as well. I have yet to test this myself.

Drupal 7: Runs smoothly, I've encountered no problems what so ever, largely because Drupal 7 was developed more with PHP 5.3 in mind, and any problems discovered on 5.4 have long since been fixed.

Drupal 8: Requires 5.3 from the start, so will work well. I will update the answer when I've tested it myself.

Updates to letharion's answer:

• Drupal 8 now requires PHP 5.4.2 (probably even higher by the time we release it), and 5.5 is supported, although not required
• Drupal 6 support for 5.3 is already good although issues still remain, handled as bugs because D8 migration from D6 will rely on PHP 5.3 being supported by D6.