You must remember that web hosts cannot always upgrade to the latest version as many accounts on the server may rely on features not available in the new versions.
Yeah! Like the other scripts, for example the drupal doesn't support PHP 5.3 version. Hope you understand it. Because there's some popular web scripts does support previous version of PHP. If you may want to code PHP 5.3 try some local server such as Wampserver. So that you may practice it.