p1.php

<?php
opcache_compile_file('p2.php');

p2.php

<?php
  
class A {
    function foo(): ?int { return null; }
}
class B extends A {
    const X = UNRESOLVED;
}
class C extends B {
    const X = 42;

    function foo(): ?int { return null; }
}

p3.php

<?php
  
class B extends A {
    function foo(): int { return 24; }
}

$c = new C;
var_dump($c->foo());