PHP 5.4 gets security support only until 14 Sep 2015. Source: http://php.net/supported-versions.php PHP 5.5 goes into security support only tomorrow.

Dropping support for PHP 5.3 is planned for Piwik 3.0 and was announced here: http://piwik.org/blog/2014/10/announcing-piwik-will-end-php-5-3-support-six-months-may-2015/
refs #7323 Drop PHP 5.3 support, Require PHP 5.4

If we release Piwik 3.0 after September 2015 (what we will most likely do), it might be worth dropping support for PHP 5.4 as well. Problem is we haven't announced it in advance as we did with PHP 5.3. It would be still worth considering it and maybe we could just announce it and drop support for PHP 5.4 in Piwik 3.X (eg 6 months after the blog post). Users using PHP 5.4 should still be able to use a previous Piwik 3.X version but the problem is they wouldn't receive our security patches anymore. Realistically it will take us 3+ months till Piwik 3.0 will be released so we would still give notice at least some time in advance but 6 months would be certainly better.

This should be part of our security policy but it will also allow us to use new features: http://php.net/manual/en/migration55.new-features.php. I'm not sure if there are eg some Ubuntu LTS versions or so that still support PHP 5.4 for a while?

I know many won't like it and we will most likely wait for another year but I created it though as we will drop support at some point anyway (and if it takes 1 or 2 years).