PHP Is Drunk

a.php

<?php

class a {
    const a = b::c;
    const b = b::d;
}

b.php

<?php
class b {
    const c = a::b;
    const d = 1;
}

test.php

<?php

spl_autoload_register(function($class) {
    require_once __DIR__ . "/" . strtolower($class) . ".php";
});

var_dump(A::a, A::b, B::c, B::d);

BTW, that outputs:

int(1)
int(1)
int(1)
int(1)