bancon

$cycle_length = 2 * ($friends - 1);
$effective_time = $time % $cycle_length;

$current = 1;
$direction = 1;

for ($i = 0; $i < $effective_time; $i++) {
    $current += $direction;

    if ($current > $friends) {
        $direction = -1;
        $current = $friends - 1;
    } elseif ($current < 1) {
        $direction = 1;
        $current = 2;
    }
}

return [$current - $direction, $current];

add $effective_time == 0 = $cycle_length *2