PHP

Diferença entre require - require_once - include - include_once
http://www.ogenial.com.br/blog/diferenca-entre-comandos-de-inclusao/
http://www.videoaulasbrasil.com.br/php/diferencas-entre-include-include_once-require-e-require_once-no-php/
http://webpub.wordpress.com/2007/11/11/diferenca-entre-require-require_once-include-include_once/
http://www.php.net/manual/en/function.require.php
http://www.php.net/manual/en/function.require-once.php
http://www.php.net/manual/en/function.include.php
http://php.net/manual/en/function.include-once.php

Alguns links interessantes

http://www.w3schools.com/php/default.asp