To this point, every script in the book has consisted of a single file that contains all of the required HTML and PHP code. But as you develop more complex Web sites, you’ll see that this methodology has many limitations. PHP can readily make use of external files, a capability that allows you to divide your scripts into their distinct parts. Frequently you will use external files to extract your HTML from your PHP or to separate out commonly used processes.
PHP has four functions for using external files: include(), include_once(), require(), and require_once(). To use them, your PHP script would have a line like
Using any one of these functions has the ...