A private function is a method defined inside a class and can only be called by that class. In PHP, private functions can be created by using the private keyword, for example: class MyClass { private function myPrivateMethod() { echo "This is a private method."; }}; Private functions cannot be called directly through object instances, nor can they be inherited by subclasses; common uses include encapsulating internal logic, assisting public methods to complete tasks, and preventing mis-calls; the difference between access modifiers is that public can be called externally, protected allows classes and subclass calls, while private is limited to internal use of the class that defines it.

Creating a private function in PHP is actually quite straightforward, just understand the concepts of classes and access modifiers. If you need to restrict a method to be used only within the class in object-oriented programming, you should use the private keyword to define this function.

What are private functions?

A private function is a method that can only be called inside the class that defines it. It cannot be accessed by external code or subclasses. This is useful when encapsulating logic, protecting data, or implementing details.

For example, suppose you have a class that processes user information, some methods are only used to assist other methods in completing tasks and should not be exposed to external calls. At this time, these methods can be set to private .

 class User {
    private function validateEmail($email) {
        return filter_var($email, FILTER_VALIDATE_EMAIL);
    }

    public function setEmail($email) {
        if ($this->validateEmail($email)) {
            $this->email = $email;
        } else {
            echo "Invalid email.";
        }
    }
}

In this example, validateEmail() is a private function that can only be called inside User class.

How to correctly declare private functions?

To declare a private function, you only need to prepend the method with private modifier:

 class MyClass {
    private function myPrivateMethod() {
        echo "This is a private method.";
    }
}

What should be noted is:

• Private methods cannot be called directly through object instances, such as $obj->myPrivateMethod() will report an error.
• Subclasses cannot inherit the private methods of the parent class.

Common scenarios for using private functions

1. Encapsulate internal logic
   For example, database connections, data verification, logging, etc. do not want to be exposed to external functions.

2. Assist public methods to complete tasks
   For example, a public method is responsible for the processing process, while several private methods are responsible for different small steps.

3. Prevent miscalculated calls
   If you don't want others to call a method at will, set it to private.

Tips: private vs protected vs public

• public : Anyone can call it.
• protected : Only the class itself and subclasses can be called.
• private : Only the class that defines it can be called.

This is very important when designing class structures. The rational use of access control can improve the security and maintainability of the code.

Basically this is what it is. Private functions, although simple, play an important role in good OOP design.

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