Use the current stable version (7.0)

If you are getting started with PHP, start with the current stable release of PHP 7.0. PHP 7.0 is very new, and adds many amazing new features over the older 5.x versions. The engine has been largely re-written, and PHP is now even quicker than older versions.

Most commonly in the near future you will find PHP 5.x being used, and the latest 5.x version is 5.6. This is not a bad option, but you should try to upgrade to the latest stable quickly – PHP 5.6 will not receive security updates beyond 2018. Upgrading is really quite easy, as there are not many backwards compatibility breaks. If you are not sure which version a function or feature is in, you can check the PHP documentation on the php.net website.

Built-in web server

With PHP 5.4 or newer, you can start learning PHP without installing and configuring a full-fledged web server. To start the server, run the following command from your terminal in your project’s web root:

> php -S localhost:8000

Source: http://www.phptherightway.com/