3. Including Code

Anywhere you are unconditionally including a class file, use require_once. Anywhere you are conditionally including a class file (for example, factory methods), use include_once. Either of these will ensure that class files are included only once. They share the same file list, so you don't need to worry about mixing them - a file included with require_once will not be included again by include_once.

Note:

include_once and require_once are statements, not functions. Parentheses should not surround the subject filename.

Example:

<?php
require_once 'ClassExample1.php';

if (condition) {
    include_once 'ClassExample2.php';
}
?>