I've had to jump through hoops recently to update my Android from 7 to 8, because Slack announced that it was going to stop supporting it starting from September.

Mind you, the last Android 7 version was released in Q1 2017, while the last Android 8 version is Q4 2017.

That's a nice explanation, however, there is already a boost candidate library: https://github.com/krzysztof-jusiak/di

But why? Because of the Dostoevsky leitmotif?

But std::unordered_map is. I wonder what would be the actual C++ memory consumption using that.

C++ unordered map does not maintain the order of the elements, which PHP does. So it is not a fair comparison.

Something approximately equivalent would be Boosts multi_index with a couple of indexes.

    using php_array_t = multi_index_container<
        int64_t,
        indexed_by<
            random_access<>,
            hashed_unique<identity<int64_t>>
        >
    >;

There are so many design considerations at play though that such a comparison would be pointless.