abyrvalg

$arr = \json_decode($a, true);

$reducer = function ($carry, $item): float {
    if ('qw_wallet_rub' === $item['alias'] ?? null) {
        return $carry + $item['balance']['amount'] ?? 0.0;
    }
    return (float)$carry;
};

$result = \array_reduce($arr['accounts'], $reducer);