PHP extensions

Class inheritance should pass extends To achieve

class  Subclass name  extends  Parent class name  {…………}

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Subclasses access the parent class public Member method

• Subclasses can inherit the constructor of the parent class , When subclasses are instantiated ,php Will first find the constructor in the subclass .
• If subclass has its own constructor ,php The constructor in the subclass will be called first .
• When a subclass does not have its own constructor ,php Will call the constructor in the parent class .

<?php
    class Website{
        public $name, $url, $title;
        public function __construct(){
            echo '-- Constructor in base class --';
        }
        public function demo(){
            echo '-- Member methods in the base class ';
        }
    }

    class ClassOne extends Website{

    }

    class ClassTwo extends Website{
        public function __construct(){
            echo '-- Constructors in subclasses --';
        }
    }

    $object = new ClassOne();
    $object->demo();
     /* The result is ：-- Constructor in base class --
    -- Member methods in the base class */

    $object2 = new ClassTwo();
    $object2->demo();
    /* The result is ：-- Constructors in subclasses --
    -- Member methods in the base class */
?>

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Subclasses access the parent class protected Member method

• Some classes inherit properties , Don't want to be accessed outside the class , This member is declared a protected member , use protected modification .
• Protected members cannot be accessed outside the class , But it can be accessed inside subclasses .
• That is to say, we can set a function in the subclass , To access this protected member in the parent class .

<?php
    class Website{
        public $name, $url, $title;
        public function __construct(){
            echo '-- Constructor in base class --';
        }
        protected function demo(){
            echo '-- Member methods in the base class ';
        }
    }

    class ClassOne extends Website{

    }

    class ClassTwo extends Website{
        public function __construct(){
            echo '-- Constructors in subclasses --';
        }

        public function test(){
            $this->demo();
        }
    }

    $object = new ClassOne();    /* The result is ：-- Constructor in base class --*/  
    //$object->demo();  // Call the parent class in the subclass to use protected The decorated member method will report an error 

    $object2 = new ClassTwo();
    $object2->test();

    /* The result is ：-- Constructors in subclasses --
    -- Member methods in the base class - */
?>

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Subclasses access the parent class private Member method

• private Decorate private members
• Private members in the parent class will not be inherited by subclasses .
• Therefore, you cannot be asked by subclasses .

<?php
    class Website{
        public $name, $url, $title;
        public function __construct(){
            echo '-- Constructor in base class --';
        }
        private function demo(){
            echo '-- Member methods in the base class ';
        }
    }

    class ClassTwo extends Website{
        public function test(){
            $this->demo();
        }
    }

    
    $object2 = new ClassTwo();
    $object2->test();
    
    /* The result is ： -- Constructor in base class --
     PHP Fatal error:  Uncaught Error: Call to private method Website::demo() from context 'ClassTwo' in 5_php_extends_private.php:14*/
     /* Cause of error ：  Call in the subclass and use in the parent class private Keyword decorated members , The program will report an error and terminate .*/
?>