This powerful language allows the creation of many functions, such as the creation of graphics, the use of Google Translation to translate texts, the authentication of users with Facebook Connect, to program searches in MySQL databases, the creation of polls with graphics that will show the results on the website, etc.
PHP 5 is one of the most secure and complete versions to the date. It includes Zend Engine 2 as a development and runtime engine, thus offering better support for object oriented programing and for MySQL and XML, as well as optimizing the performance.
To sum up, the use of PHP is essential to develop dynamic web applications that require accessing the data contained within any database. Therefore, if this is your case, it's advisable to download PHP 5.