Some versions of PHP are not compatible with certain versions of web servers, at least in Apache. I have next to zero experience in nginx, but if I'm not mistaken it's more commonly used for CGI, being event-based and not process-based. CGI still has it's place for some things, but I've found using MySQL more secure, and MSSQL more secure still.
On that note, you might prefer installing Apache over nginx, unless you have specific reason otherwise: I'm not sure how well developed nginx is on the RasPi, but I know many people use Apache to run web servers on it. If you're sure about using nginx, make sure there's no errors when you run the line from that page [sudo apt-get install php5-fpm].
Your issue itself seems that the PHP is not parsing. HTML will process and HTML between carrots (< and >), and not show that output to the user. However, unless PHP is working, it still is visible if you view source. To make sure PHP is installed, view the page source and see if you can see that php code. If you can, PHP isn't installed or working. If it doesn't show up, there's likely a problem with the compatibility of the server. Also, some PHP code ends up deprecated on newer versions, while some instructions are new. Make sure your issue is you're using correct code/syntax for your installed PHP version.
Lastly, but possibly most importantly: Make sure you're following the "TEST THE WEB SERVER" of your tutorial properly, and visit it from ANOTHER computer. If you try from the same IP address, it can result in loopback, where PHP won't parse for the same IP address as the source.
I also found this resource that might lead you in the right direction.
https://serverfault.com/questions/593677/php-file-are-not-parsed-by-fastcgi-in-nginx
I know this is a long answer, but it's a good way to troubleshoot with logic and understanding of how it works.