syntax
  variables
  strings
  operators
  conditionals
  switches
  arrays
  loops
  functions
  form input using $_POST
  url input using $_GET
  
If you know HTML and you want to extend your web design knowledge, php is the next logical step. Almost every server now supports php, its very simple to learn, and very versatile. 

Any file that contains php code must be named with the .php extension, the .php extension is how your web server knows that it should check for php code. PHP code can be put anywhere inside a normal html document, however, all php code must be enclosed by a php declaration block:
<?php

?>

Comments:
It is very important to comment your php code, so when you or someone else goes back to it later, you can easily identify what exactly it does. Here are two ways to comment your code.

Two forward slashes // is used for single line comments.
Example:
<?php

// This is a comment on one line

?>

if you want to have a multi line comment you would start it with /* and end it with */
Example:
<?php

/* This is
is a multi
line comment */

?>


Code Syntax
Not much is required in a line of php code, the only thing you need to make sure to add is a semicolon ; at the end of every code line. This tells php that it can go on to the next bit of code. 
Example:

<?php

$name="Bob";
print "Hello $name"; //Outputs "Hello Bob"

?>

The example above has two lines of code, each ends with a semicolon. The first line puts the word "Bob" into a string variable we called $name, the second line prints "Hello Bob".