Open up your favorite text editor. The simpler the better; TextEdit or WordPad are better than using Microsoft Word. Begin your document with <?php and end it with ?>. Now go to File >> Save As…, type in a name, and save it to your desktop. Now close the text editor, navigate to that file on your desktop, and make sure it ends with “.php” instead of “.txt” — you can simply rename it and that should take care of things.