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.
