<?php

abstract class A
{
    static $lalka;
    static function getLalka()
    {
        return static::$lalka;
    }
    static function setLalka($lal)
    {
        static::$lalka = $lal;
    }
}

class B extends A {}

class C extends A {}

B::setLalka('sasai');
C::setLalka('yoba');

var_dump(B::getLalka(), C::getLalka());