The require_once() statement includes and evaluates
the specified file during the execution of the script.
This is a behavior similar to the require() statement,
with the only difference being that if the code from a file has already
been included, it will not be included again. See the documentation for
require() for more information on how this statement
require_once() should be used in cases where
the same file might be included and evaluated more than once during a
particular execution of a script, and you want to be sure that it is
included exactly once to avoid problems with function redefinitions,
variable value reassignments, etc.
For examples on using require_once() and
include_once(), look at the
PEAR code included in the
latest PHP source code distributions.
Be aware, that the behaviour of require_once()
and include_once() may not be what you expect
on a non case sensitive operating system (such as Windows).
Example 11-8. require_once() is case insensitive on Windows
require_once("a.php"); // this will include a.php
require_once("A.php"); // this will include a.php again on Windows!
versions of PHP prior to PHP 4.3.0 do not
support accessing remote files via this function, even if
allow_url_fopen is enabled.
See also require(),
get_included_files(), readfile(), and