PHP 5.3 has now entered end-of-life; no further releases for PHP 5.3 will be made by the PHP group (http://php.net/eol.php).
As such, Simba Hosting plan to upgrade from PHP 5.3 to PHP 5.4 in October of this year. (PHP is the programming language that most websites – including WordPress and Joomla ones – are created in).
Since the first version of PHP 5.4 was released in March 2012, no vaguely modern website should have any upgrade issues with it. However, if your website was built before March 2012, and has not been kept up to date since, then you may want to take a look – have a chat with your web developer before the end of the month to see what, if any, minor issues you may need to look at to keep up.