Differences between require and include, include_once and require_once

The include() statement includes and evaluates the specified file.The documentation below also applies to require(). The two constructs are identical in every way except how they handle failure. include() produces a Warning while require() results in a Fatal Error. In other words, use require() if you want a missing file to halt processing of the page.
include() does not behave this way, the script will continue regardless.

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. As the name suggests, it will be included just once.include_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 redefinition's, variable value reassignments, etc.

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.