Since DPCalendar version 3.0, which was released in February 2013, we no longer support PHP 5.2, please let us explain why we decided to drop PHP 5.2 support.
PHP 5.2 became end-of-life on January 6th, 2011 when the final version, PHP 5.2.17, was released. As the PHP website reads on that page:
If you are using these releases, you are strongly urged to upgrade to a current version, as using older versions may expose you to security vulnerabilities and bugs that have been fixed in more recent versions of PHP.
Security risk
This is exactly the case with PHP 5.2, a version of PHP which is end of life for over 2 years now. As a matter of fact, just a mere days after it became end of life a major security flaw was detected. It was fixed in PHP 5.3 but not in PHP 5.2. If you are using PHP 5.2 you are exposed to known security threats which are being used to attack web sites. The nature of these attacks means that your server's firewall and/or other security tools cannot protect your site. If you are using PHP 5.2 your site will be hacked. The question is not if, but when. So, using PHP 5.2 hurts your site's security. We are not joking!!!
PHP 5.3 is approaching the end-of-life mark itself, whereas PHP 5.4 is now officially considered mature. So, using PHP 5.2 is, practically, being two major releases behind, in the software timescale that's an eternity.
What happens now when my host is still on PHP 5.2?
There are hosts which still offer PHP 5.2 by default but do support PHP 5.3 or 5.4 as an option. On these hosts you can ask them for the proper way to upgrade to PHP 5.3. Usually it's a configuration option in your site's control panel or in your .htaccess. Please do. Joomla! 2.5 and all of its extensions are PHP 5.3 ready. Joomla! 3.0 and later actually requires PHP 5.3, you can't use it on older PHP versions.
There are still hosts which do not offer you any way to use PHP 5.3 or later. These hosts must be avoided at all cost. It means that the hosting company doesn't care about investing the minimal amount of time required to properly setup their machines and ensure your security. Do you really want to trust them with your site? If you're stuck on such a host we strongly recommending moving to a decent hosting company.