include
include_once
Control Structures
PHP Manual

## require_once

(PHP 4, PHP 5)

The require_once statement is identical to require except PHP will check if the file has already been included, and if so, not include (require) it again.

See the include_once documentation for information about the _once behaviour, and how it differs from its non _once siblings.

include
include_once
Control Structures
PHP Manual