<?php

abstract class CrossVersionA {
    private static $prop;
    
    public static function bar() {
        // Refering to a `private static` property using `self::` is fine.
        self::$prop = 10;
    }
}

class ChildOkayA extends CrossVersionA {
    public static function bar() {
        parent::bar();
    }
}

abstract class CrossVersionB {
    protected static $prop;
    
    public static function bar() {
        // Refering to a `protected static` property using `static::` is fine.
        static::$prop = 10;
    }
}

class ChildOkayB extends CrossVersionB {
    public static function bar() {
        parent::bar();
    }
}

class SometimesCrossVersion {
    private static $prop;
    
    public static function bar() {
        // Refering to a `private static` property using `static::` is fine if the method is not called from a child class.
        static::$prop = 10;
    }
}

abstract class NotCrossVersionB {
    private static $prop;
    
    public static function bar() {
        // Refering to a `private static` property using `static::` is problematic.
        static::$prop = 10;
    }
}

class ChildError extends NotCrossVersionB {
    public static function bar() {
        parent::bar();
    }
}

ChildOkayA::bar(); // This will work fine.
ChildOkayB::bar(); // This will work fine.
SometimesCrossVersion::bar(); // This will work fine as long as the class is not extended and the method is not called from a child class.
ChildError::bar(); // This will only work in PHP 7.4+.