是否可以像在Java中那样在PHP中访问隐藏的实例字段？

Question

In Java as in PHP, instance fields declared in parent classes are inherited in child classes. Therefore, it is possible to access them using the keyword this (resp. $this in PHP).

In Java, if the child class happens to declare a field with the same name as the parent class, the parent class's field is hidden (see JLS 8.3.1.1-3: Hiding of Instance Variables ). It is there, but if the child class wants to access it, it needs to use the keyword super , as in the following example:

class Foo {
    protected int x = 1;
}

class Bar extends Foo {
    protected int x = 2;

    void somemethod() {
        System.out.println(this.x); // prints 2
        System.out.println(super.x); // prints 1
    }
}

Please note that I consider hiding fields as a bad practice, as it only makes for confusing code and easily leads to logic bugs. It is best not to declare instance fields with the same name as an instance field of a parent class. Therefore, this question is of a purely academic nature.

Nevertheless I am curious: Is it in principle possible to access hidden instance fields in PHP, as is possible in Java? Would it be possible to translate the above code snippet to PHP?

<?php

class Foo {
    protected $x = 1;
}

class Bar extends Foo {
    protected $x = 2;

    function somemethod() {
        echo $this->x, PHP_EOL; // prints 2
        //echo $parent->x, PHP_EOL; // doesn't compile... is there a way to do it?
    }
}

Please note that I am talking about instance variables here. Class (ie, static) variables are a different story altogether and are not a subject of this question.

If it is not possible: Does this mean that the parent instance field effectively gets overwritten in PHP (instead of just hidden), and is thus inaccessible?

Answer 1

The reason why this is not working for you, is because you are overriding the parent. Foo->$x is available in the child scope, therefor when you define X as 2 in Bar it overrides the previous set variable.

If you want something like this to work like in Java, then you would have to use constants.

class Foo {
    const x = 1;
}

class Bar extends Foo {
    const x = 2;

    function somemethod() {
        echo parent::x; // prints 1
        echo self::x; // prints 2
    }
}

Or you could simply change the variable name in Bar and then you would have direct access to it through inheritance.