PHP: static variable vs static field

index.php

<?php

// https://3v4l.org/ZoCO0

class X
{
    static $y = 0;
    static $z = 0;
    
    public static function test()
    {
        static $x = 0;
        echo get_called_class() . ": " . $x . " | " . static::$y . " | " . self::$z . "\n";
        $x++;
        static::$y++;
        self::$z++;
    }
}

class A extends X
{
    public function do1()
    {
        self::test();
    }

    public static function do2()
    {
        self::test();
    }
    
    public function do3()
    {
        static::test();
    }

    public static function do4()
    {
        static::test();
    }
}

class B extends X
{
    public function do1()
    {
        self::test();
    }

    public static function do2()
    {
        self::test();
    }
    
    public function do3()
    {
        static::test();
    }

    public static function do4()
    {
        static::test();
    }
}

$a1 = new A();
$a2 = new A();
$b1 = new B();
$b2 = new B();

$a1->do1();
$a2->do1();
$a1->do3();
$a2->do3();
$b1->do1();
$b2->do1();
$b1->do3();
$b2->do3();
A::do2();
A::do4();
B::do2();
B::do4();