What is difference between include,require,include_once and require_once() ?

• Include and require both uses to include a PHP page within another PHP page in a project to initialize some predefined functions or programs. But the only difference between both is that if you have
  used include(test.php) and test.php does not exist then you would receive a warning error but
  execution would not interrupt while, if you have used require(test.php) and test.php does not
  exist then you would receive a fatal error and execution would be interrupted.
• Same thing happens with include_once and require_once except that in that case file is required
  only once.

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