Detailed explanation of the definition, usage, and difference examples of public, private, and protected attributes in PHP

public: Public attributes or methods

can be called in subclasses through self::var or self::method, and can be called in parent classes through parent::method method, but public properties cannot be called.

In the instance, it can be called through $obj->var or self::method

protected: protected type

In the subclass, you can call it through self::var or self::method. You can call the method in the parent class through parent::method.
In the instance, you cannot call the protected type method through $obj->var. Or attribute

private: private type

The attributes or methods of this type can only be used in this class, and cannot be called in instances of this class, subclasses, or instances of subclasses Properties and methods of private types

2. The difference between self and parent
a). These two objects are commonly used in subclasses. Their main difference is that self can call public or protected properties in the parent class, but parent cannot call

b).self:: It represents the static members of the current class ( Methods and properties) Unlike $this, $this refers to the current object

<?php
class BaseClass {
  public $public = &#39;public&#39;;
  private $private = &#39;private&#39;;
  protected $protected = &#39;protected&#39;;
  function construct(){
  }
  function print_var(){
    print $this->public;echo &#39;<br />&#39;;
    print $this->private; echo &#39;<br />&#39;;
    print $this->protected; echo &#39;<br />&#39;;
  }
}

class Subclass extends BaseClass {
  // public $public = &#39;public2&#39;;
  protected $protected =&#39;protected2&#39;;
  function construct(){
    echo $this->protected;//可以访问，因为类中定义为受保护的，所以在本类或子类中可以，子类中还可以重复付值
    echo &#39;<br />&#39;;
    echo $this->private;//error 因为是私有的只有在定义她的类baseclass中可以用
  }
}
  $obj1 = new BaseClass();
  $obj1->print_var();
  //echo $obj1->protected;//error 因为是受保护的，只有在本类内部或子类父类中可以调用
  //echo $obj1->private;//error 同上私有的，只有在本类内调用
  echo $obj1->public;
  echo "<hr />";
  $obj2 = new Subclass();
  echo &#39;<br />&#39;;
  echo $obj2->public;echo &#39;<br />&#39;;
  echo $obj2->protected;
  //echo $obj2->private;//error
  //echo $obj2->protected;//error
?>

Summary

public means global, and can be accessed by both internal and external subclasses of the class;
private means private, It can only be used within this class;
protected means protected and can only be accessed in this class or subclass or parent class;

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