子班影响父母班

Question

Here is my code:

class Parent1
{
    static $db = null;

    public function __construct()
    {
        self::$db = 'a';
    }
}

class Child extends Parent1
{

    public function __construct()
    {
        parent::__construct();
        self::$db = 'b';
    }
}

$myParent = new Parent1();
echo $myParent::$db; //"a"

$myChild = new Child();
echo $myChild::$db; //"b"
echo $myParent::$db; //"b" it should be "a"

Why $myParent::$db is changing to b ? How to prevent it??

Answer 1

Why?

static $db = null;

$db is static , it's not linked to the instance.
self::$db = 'b'; will change the unique and shared instance of $db .

How to prevent it?

You can't. It's how static fields work.
By the way, calling static from an instance ( $aa::field ) is not a good think.

Take a look at the documentation about static in PHP because you probably don't understand how it work.

Answer 2

You are using static variables. These are class level and shared across all instances. You might want to change them to instance variables... see below.

<?php
class Parent1
{
    public $db = null;

    public function __construct()
    {
        $this->db = 'a';
    }
}

class Child extends Parent1
{

    public function __construct()
    {
        parent::__construct();
        $this->db = 'b';
    }
}

However, writing to $myChild->db WILL change the variable from the parent because it is an inherited variable but it won't affect the $db value from $myParent.

Answer 3

I found solution. I redeclare static in Child - now it works. Thanks for explaining static

class Parent1
{
    static $db = null;

    public function __construct()
    {
        self::$db = 'a';
    }
}

class Child extends Parent1
{
    static $db = null;

    public function __construct()
    {
        parent::__construct();
        self:$db=parent::$db;
        self::$db = 'b';
    }
}

$myParent = new Parent1();
echo $myParent::$db; //"a"

$myChild = new Child();
echo $myChild::$db; //"b"
echo $myParent::$db; //"a" => as it should