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