This is a demo-application on Google App Engine that is written in PHP instead of Java, Python or Go. To be able to do so, it uses Quercus, a Java implentation of PHP5.
The only PHP in this page is just printing out the current date/timestamp and that goes a little like this:
Monday 24th of July 2017 06:38:35 AMOn this page you can admire the output of phpinfo() as well.
If you want create a PHP app on GAE as well, you can download an app engine project template for PHP applications here, which includes the basic configuration and the Quercus jar to run your PHP-code.
More info on how to use the template to create your very first GAE PHP app can be found on my blog.