What exactly are late static bindings in PHP?

What exactly are late stati开发者_StackOverflow社区c bindings in PHP?

You definitely need to read Late Static Bindings in the PHP manual. However, I'll try to give you a quick summary.

Basically, it boils down to the fact that the self keyword does not follow the same rules of inheritance. self always resolves to the class in which it is used. This means that if you make a method in a parent class and call it from a child class, self will not reference the child as you might expect.

Late static binding introduces a new use for the static keyword, which addresses this particular shortcoming. When you use static, it represents the class where you first use it, ie. it 'binds' to the runtime class.

Those are the two basic concepts behind it. The way self, parent and static operate when static is in play can be subtle, so rather than go in to more detail, I'd strongly recommend that you study the manual page examples. Once you understand the basics of each keyword, the examples are quite necessary to see what kind of results you're going to get.

From PHP: Late Static Bindings - Manual:

As of PHP 5.3.0, PHP implements a feature called late static binding which can be used to reference the called class in the context of static inheritance.

Late static binding tries to solve that limitation by introducing a keyword that references the class that was initially called at runtime. ... It was decided not to introduce a new keyword, but rather use static that was already reserved.

Let's see an example:

<?php
    class Car
    {
        public static function run()
        {
            return static::getName();
        }

        private static function getName()
        {
            return 'Car';
        }
    }

    class Toyota extends Car
    {
        public static function getName()
        {
            return 'Toyota';
        }
    }

    echo Car::run(); // Output: Car
    echo Toyota::run(); // Output: Toyota
?>

Late static bindings work by storing the class named in the last "non-forwarding call". In case of static method calls, this is the class explicitly named (usually the one on the left of the :: operator); in case of non-static method calls, it is the class of the object. A "forwarding call" is a static one that is introduced by self::, parent::, static::, or, if going up in the class hierarchy, forward_static_call(). The function get_called_class() can be used to retrieve a string with the name of the called class and static:: introduces its scope.

There is not very obvious behavior:

The following code produces 'alphabeta'.

class alpha {

    function classname(){
        return __CLASS__;
    }

    function selfname(){
        return self::classname();
    }

    function staticname(){
        return static::classname();
    }
}

class beta extends alpha {

    function classname(){
        return __CLASS__;
    }
}

$beta = new beta();
echo $beta->selfname(); // Output: alpha
echo $beta->staticname(); // Output: beta

However, if we remove the declaration of the classname function from the beta class, we get 'alphaalpha' as the result.

I'm quoting from the book: "PHP Master write cutting-edge code".

Late static binding was a feature introduced with php 5.3. It allows us to inherit static methods from a parent class, and to reference the child class being called.

This means you can have an abstract class with static methods, and reference the child class's concrete implementations by using the static::method() notation instead of the self::method().

Feel free to take a look at the official php documentation as well: http://php.net/manual/en/language.oop5.late-static-bindings.php

The clearest way to explain Late Static Binding is with a practicle example. I'm using it in a Template method pattern. See below.

abstract class AbstractTemplate {
    
    public const AWESOME_LIST = [''];
    
    public function someFunction(): void {
        $awesomeList = $this->getAwesomeList();

        // OUTPUT: ['harry','henk','john'];
        var_dump($awesomeList); 
    }


    /**
     * This function gets static constants from CHILD classes
     */
    public function getAwesomeList(): array
    {
        return static::AWESOME_LIST;
    }
}

class ConcreteTemplate extends AbstractTemplate {
    
    public const AWESOME_LIST = ['harry','henk','john'];
    
    public function someFunction(): void {
        parent::someFunction();
    }
}

$concreteTemplate = new ConcreteTemplate();
$concreteTemplate->someFunction();

Notice the static keyword in method getAwesomeList. Let's change a bit now:

public function getAwesomeList(): array
{
    return self::AWESOME_LIST;
}

The output of the var_dump at someFunction would be:

array (size=1)
  0 => string '' (length=0)

The static keyword is used in a Singleton design pattern. See link: https://refactoring.guru/design-patterns/singleton/php/example

The simplest example to show the difference.
Note, self::$c

class A
{
    static $c = 7;

    public static function getVal()
    {
        return self::$c;
    }
}

class B extends A
{
    static $c = 8;
}

B::getVal(); // 7

Late static binding, note static::$c

class A
{
    static $c = 7;

    public static function getVal()
    {
        return static::$c;
    }
}

class B extends A
{
    static $c = 8;
}

B::getVal(); // 8

Looking at it from a "why would I use this?" perspective, it's basically a way to change the context from which the static method is being interpreted/run.

With self, the context is the one where you defined the method originally. With static, it's the one you're calling it from.

For example:

abstract class Builder {
    public static function build() {
        return new static;
    }
}

class Member extends Builder {
    public function who_am_i() {
         echo 'Member';
    }
}

Member::build()->who_am_i();

Also, watch if you update static variables in child classes. I found this (somewhat) unexpected result where child B updates child C:

class A{
    protected static $things;
}

class B extends A {
    public static function things(){
        static::$things[1] = 'Thing B';
        return static::$things; 
    }
}

class C extends A{
    public static function things(){
        static::$things[2] = 'Thing C';
        return static::$things;        
    }
}

print_r(C::things());
// Array (
//   [2] => Thing C
// )

B::things();

print_r(C::things()); 
// Array (
//    [2] => Thing C
//    [1] => Thing B
// )

You can fix it by declaring the same variable in each child class, for example:

class C extends A{
    protected static $things; // add this and B will not interfere!

    public static function things(){
        static::$things[2] = 'Thing C';
        return static::$things;        
    }
}