To start programming in PHP you need to have a text editor such as notepad, notepad++ for MS Windows users, GEdit, Vi, Nano, Emacs for Linux. You will also need to understand at least HTML. If you have that already, it's now appropriate to begin by understanding the PHP Syntax.
You will also need to have a good PHP Server. A good package already exists for this. Check XAMPP, WAMP, LAMP, and MAMP servers; where XAMPP is for all modern operating systems, WAMP is for windows, LAMP is for Linux, and MAMP is for Macintosh. Check
Apache Friends website for your preferred Server.
<?php
?>
This illustrates the opening and closing tag for all php scripts.
Then you need to understand the data types supported by php.
Since you are new to php, I would refer you to the official documentation for PHP at
http://www.php.net/docs.php