From Lunarpages Web Hosting Wiki
How can I gather more information about the version of PHP that is installed on my hosting account?
All you need to do is create a new empty file, with any name. As an example, lets call it testing.php. The name of the file must end with the .php extension. Next, you will want to include this code inside of the testing.php file:
<?php phpinfo(); ?>
Now upload that file to your hosting account (if not already there) and then navigate yourself to it via your browser. What is this phpinfo command all about? Here is more information from PHP.net:
Outputs a large amount of information about the current state of PHP. This includes information about PHP compilation options and extensions, the PHP version, server information and environment (if compiled as a module), the PHP environment, OS version information, paths, master and local values of configuration options, HTTP headers, and the PHP License.
Now you can gather more information about the version of PHP you are using.