What are the differences between require and include, include_once?

require_once() and include_once() are both the functions to include and evaluate the
specified file only once. If the specified file is included previous to the present call
occurrence, it will not be done again.

But require() and include() will do it as many times they are asked to do.


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. The major difference between include() and require() is that in failure
include() produces a warning message whereas require() produces a fatal errors.

All three are used to an include file into the current page.
If the file is not present, require(), calls a fatal error, while in include() does not.
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. It des not call a fatal error if file not exists. require_once() does the same
as include_once(), but it calls a fatal error if file not exists.

File will not be included more than once. If we want to include a file once only and
further calling of the file will be ignored then we have to use the PHP function
include_once(). This will prevent problems with function redefinitions, variable value
reassignments, etc.