在PHP上通过自身名称或类名称访问常量之间的区别？

Question

What is the difference, if any, between accessing a constant using self vs class name.

class MyClass {

    const MAGIC_NUMBER = 4;

    protected $number;

    public function __construct()
    {
        $this->number = self::$MAGIC_NUMBER;
    }
}

vs

class MyClass {

    const MAGIC_NUMBER = 4;

    protected $number;

    public function __construct()
    {
        $this->number = MyClass::MAGIC_NUMBER;
    }
}

Answer 1

There is no difference between them.
Also you should not put $ there:

const MAGIC_NUMBER = 4;
protected $number;

Answer 2

The examples you gave are invalid, and I think you might be confusing constants and static variables.

Class constants:

class MyClass {
    const MAGIC_NUMBER = 4;

    protected $number;

    public function __construct()
    {
        $this->number = self::MAGIC_NUMBER; // or MyClass::MAGIC_NUMBER
    }
}

Static class variables:

class MyClass2 {
    static $MAGIC_NUMBER = 4;

    protected $number;

    public function __construct()
    {
        $this->number = MyClass2::$MAGIC_NUMBER; // or self::$MAGIC_NUMBER
    }
}

The main difference is that MyClass2::$MAGIC_NUMBER can be changed, while in both cases the values can be accessed without requiring an instance of the class.