At root, ``++require++``, ``++require_once++``, ``++include++``, and ``++include_once++`` all perform the same task of including one file in another. However, the way they perform that task differs, and they should not be used interchangeably.
``++require++`` includes a file but generates a fatal error if an error occurs in the process.
``++include++`` also includes a file, but generates only a warning if an error occurs.
Predictably, the difference between ``++require++`` and ``++require_once++`` is the same as the difference between ``++include++`` and ``++include_once++`` - the "_once" versions ensure that the specified file is only included once.
Because including the same file multiple times could have unpredictable results, the "once" versions are preferred.
Because ``++include_once++`` generates only warnings, it should be used only when the file is being included conditionally, i.e. when all possible error conditions have been checked beforehand.
