How to judge is not Pravite function?

How to judge is not Pravite function??
For example, the CI framework, we invoke the pravite decorated function from the controller on the URL. It will jump to the 404_error interface. How is it implemented?

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------Solution--------------------
A private method function cannot be accessed externally, and when the method that finds the request does not exist, it turns 404. It's not hard to do, is it?

$p = new Class;
if (! method_exists ($p)) {
  Header ("http/1.0 404 Not Found")
  Exit
}

No matter what the framework, you can modify it is just a callback (or plug-in) of the main control program
You don't write anything, and it doesn't affect the normal functioning of the framework.

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