Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition

Including and Requiring Files

As you progress in your use of PHP programming, you are likely to start building a library of functions that you think you will need again. You’ll also probably start using libraries created by other programmers.

There’s no need to copy and paste these functions into your code. You can save them in separate files and use commands to pull them in. There are two types of commands to perform this action: include and require.

The include Statement

Using include, you can tell PHP to fetch a particular file and load all its contents. It’s as if you pasted the included file into the current file at the insertion point. Example 5-6 shows how you would include a file called library.php.

Example 5-6. Including a PHP file

<?php
include "library.php";

// Your code goes here
?>

Using include_once

Each time you issue the include directive, it includes the requested file again, even if you’ve already inserted it. For instance, suppose that library.php contains a lot of useful functions, so you include it in your file. Now suppose you also include another library that includes library.php. Through nesting, you’ve inadvertently included library.php twice. This will produce error messages, because you’re trying to define the same constant or function multiple times. To avoid this problem, use include_once instead (see Example 5-7).

Example 5-7. Including a PHP file only once

<?php
include_once "library.php";

// Your code goes here
?>

Then, if another include or include_once for ...