PHP 5.2 is a dead release. It 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

This is exactly the case with PHP 5.2, a version of PHP which is end of life for over 4 years. 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 a security component (like Admin Tools
Professional) 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.

The current PHP versions at the time of this writing are PHP 5.6.7 and PHP 5.4.39, both released in March 2015. So, using PHP 5.2 is, practically, being three major releases behind; in the software timescale that's an eternityPHP 5.2 is also slow. Really slow. According to our tests, PHP 5.2 is six to seven times (600% - 700%) slower than PHP 5.3 on the same hardware. No joking!

Remember that search engines rank faster loading sites first. So, using PHP 5.2 hurts your search engine rankings.

How Do I Upgrade to PHP 5.3+?

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.

IMPORTANT: Some hosts claim that the only alternative to PHP 5.2 they can offer is PHP 6. DO NOT USE THAT!

PHP 6 was supposed to be the next release after PHP 5.2 but its development was halted and PHP 5.3 was released instead. PHP 6 is pre-alpha, unstable software which will break your site. If your host didn't explain this to you it's best to find a new host.

Even worse, 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.

If you need help choosing a new hosting provider, read our article on WordPress hosting providers.