公共，私有和受保护的有什么区别？ - What is the difference between public, private, and protected?

问题：

When and why should I use public , private , and protected functions and variables inside a class? 何时以及为何应该在类内使用public ， private和protected函数和变量？ What is the difference between them? 它们之间有什么区别？

Examples: 例子：

// Public
public $variable;
public function doSomething() {
  // ...
}

// Private
private $variable;
private function doSomething() {
  // ...
}

// Protected
protected $variable;
protected function doSomething() {
  // ...
}

---

解决方案：

参考一： https://stackoom.com/question/IIdd/公共-私有和受保护的有什么区别
参考二： https://oldbug.net/q/IIdd/What-is-the-difference-between-public-private-and-protected