All what you need to know about PHP from A to Z: 

PHP is a programming language that can do all sorts of things: evaluate form data sent from a browser, build custom web content to serve the browser, talk to a database, and even send and receive cookies
  • The PHP runs at the server while the JV runs only in the browser, or client
  • The PHP code is written like this:
  • <?php echo "I'm learning PHP!";   ?>
  • The file should be saved with extension  .php
  • The comments should be putted after //


All what you need to now about the  HTML 5
The HTML 5 new elements are : header, footer, section, article, nav, aside, figure, figcaption, time, main;so there is one rule all content putted outside these elements will be considered as default content. 
For better SEO indexing you will have to apply Microdata.