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.
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require includes a file but generates a fatal error if an error occurs in the process.
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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.
What is the potential impact?
Including the same file multiple times could have unpredictable results, the _once versions are preferred. Additionally, as
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.
