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 only difference being that if the code from a file has already been included, it will not be included again, and include_once returns
TRUE. As the name suggests, the file will be included just once.
include_once may be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, so in this case it may help avoid problems such as function redefinitions, variable value reassignments, etc.
See the include documentation for information about how this function works.
With PHP 4, _once functionality differs with case-insensitive operating systems (like Windows) so for example:
Example #1 include_once with a case insensitive OS in PHP 4
include_once "a.php"; // this will include a.php
include_once "A.php"; // this will include a.php again! (PHP 4 only)