Online PHP editor | output for TUbfJ

<?php declare(strict_types=1); interface I { public function example(); } class A implements I { public function example(): int { return 123; } // compatible with I::example } class B extends A { public function example(): string { return "string"; } // compatible with I::example } function accepts_i_expects_any(I $i) { var_dump($i->example()); } accepts_i_expects_any(new B); // receives string, no problem function accepts_a_expects_int(A $a) { var_dump($a->example()); } accepts_a_expects_int(new B); // receives string, problem