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 good step (In my personal opinion). Almost every server now supports php, its very simple to learn, and very versatile. You may also consider learning javascript as your next step if your knowledge is limited to HTML.

LESSON 1 - PHP SYNTAX TUTORIAL:
You can think of syntax kind of like punctuation, certain things are required at certain places in PHP, so that is where we will start.

The .php file extension
Most PHP files will contain a mixture of php as well as HTML, however, 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 Declaration Block
PHP code can be put anywhere inside a normal html document, however, all php code must be enclosed by a php declaration block:
<?php

?>

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. Almost like a period at the end of a sentence.
Example:
<?php

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

?>

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".

How to comment PHP code:
It is very important to comment your 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 PHP 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 */

?>

Lesson 2 - PHP VARIABLES