<?php

class ParentClass
{
    static $someVal = 80;

    public function getMyVal()
    {
        printf('I am %s, my $someval is %d%s',static::class, static::$someVal, PHP_EOL);
    }
}

class ChildClass1 extends ParentClass
{
    public function doIt()
    {
        $this->getMyVal();
    }
}

class ChildClass2 extends ParentClass
{
    static $someVal;

    public function doIt()
    {
        static::$someVal = 35;
        $this->getMyVal();
    }
}

$c1 = new ChildClass1();
$c1->doIt();

$c2 = new ChildClass2();
$c2->doIt();

$c1 = new ChildClass1();
$c1->doIt();

/**
I am ChildClass1, my $someval is 80
I am ChildClass2, my $someval is 35
I am ChildClass1, my $someval is 80
**/