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. You don't need parentheses around the filename to be included, however you should do it anyway and use ' (apostrophes) not " (quotes):
include('pre.php');