The difference between self and static in PHP in methods

Let's start with the last piece of code

class Father{
    public static $a = 2;

    public function selfTest(){
        var_dump(self::$a);
    }

    public function staticTest(){
        var_dump(static::$a);
    }
}

class Son extends Father {

    public static $a = 20;

}

 $obj = new Son();
 $obj->selfTest();
 $obj->staticTest();

Output ：

int(2)

int(20)

Conclusion ：self Express self Keyword class , That is to say, here self yes Father class ,self::$a Namely Father::$a, Although it is a subclass call selfTest Method self::$a It will not show polymorphism

static Represents the class that calls the method , here obj The class of the object is Son, be static::$a That means Son::$a, If obj The class of the object is Father class , be static::$a That means Father::$a, in other words static::$a Will show polymorphism