While tools like MySQL and PHP aren't as bad as the community makes them out to be - Facebook is a terrible example how "impressive" these tools can be considering that Facebook almost entirely rewrote both of those tools (https://github.com/webscalesql/webscalesql-5.6, https://github.com/facebook/hhvm)

You can do many things with PHP and MySQL but the real question: is it worth it? I would be interested comparing the TCO (including development time) with platforms like this. I think MySQL is lagging behind Postgres nowadays so I would much rather use that even for storing JSON, even though I do not think that storing JSON in an SQL engine is a particularly good idea, it might work. Our #1 complain against MySQL + JSON was the insert and update time that was significantly higher than the normal insert/update for obvious reasons. If the workload is not update heavy this approach might even work. :)