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. ]]> include_once and require_once are statements, not functions. Parentheses should not surround the subject filename. ]]> ('PHP/CodeSniffer.php'); ]]>