How to run php files on my computer

I just put the content in the question in a file called test.php and ran php test.php. (In the folder where the test.php is.)

$ php foo.php                                                                                                                                                                                                                                                                                                                                    
15

php have a easy way to run a light server:
first cd into php file directory, then

php -S 127.0.0.1:8000

then you can run php

In short:

1. Install WAMP

2. Put this file to C:\wamp\www\ProjectName\filename.php

3. Go to browser: http://localhost/ProjectName/filename.php

You have to run a web server (e.g. Apache) and browse to your localhost, mostly likely on port 80.

What you really ought to do is install an all-in-one package like XAMPP, it bundles Apache, MySQL PHP, and Perl (if you were so inclined) as well as a few other tools that work with Apache and MySQL - plus it's cross platform (that's what the 'X' in 'XAMPP' stands for).

Once you install XAMPP (and there is an installer, so it shouldn't be hard) open up the control panel for XAMPP and then click the "Start" button next to Apache - note that on applications that require a database, you'll also need to start MySQL (and you'll be able to interface with it through phpMyAdmin). Once you've started Apache, you can browse to http://localhost.

Again, regardless of whether or not you choose XAMPP (which I would recommend), you should just have to start Apache.