Slide 3.b: PHP (Hypertext Preprocessor)
Slide 4.d: PHP variables
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PHP Syntax

You cannot view the PHP source code by selecting View source in the browser—you will only see the output from the PHP file, which is plain HTML or text. This is because the scripts are executed on the server before the result is sent back to the browser.

Basic PHP Syntax
A PHP scripting block always starts with <?php and ends with ?>. A PHP scripting block can be placed anywhere in the HTML document. 
  <html><body>
  <?php

  ?>
  </body></html>
   
A PHP file normally contains HTML tags, just like an HTML file, and some PHP scripting code. We have an example of a simple PHP script which sends the text “Hello, World!” to the browser: 
  <html><body>
  <?php
    echo "Hello, World!";
  ?>
  </body></html>
   
Each code line in PHP must end with a semicolon. The semicolon is a separator and is used to distinguish one set of instructions from another. There are two basic statements to output text with PHP: echo and print. In the example above we have used the echo statement to output the text “Hello, World!”.

Comments in PHP
In PHP, we use // to make a single-line comment or /* and */ to make a large comment block. 
  <html><body>
  <?php
    //This is a comment
    /* This is
       a comment block
    */
  ?>
  </body></html>