The include_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include() statement, with the important difference that if the code from a file has already been included, it will not be included again.
As mentioned in the require_once() description, the include_once() should be used in the cases in which 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 more examples on using require_once() and include_once(), look at the PEAR code included in the latest PHP source code distributions.
include_once() was added in PHP 4.0.1pl2
See also: require(), include(), require_once(), get_required_files(), get_included_files(), readfile(), and virtual().