A Primer on Web Caching

Makes it sounds like Redis is "closer to the metal" with a lower-level API than Memcached. Also how can you be "more persistent"...you're either persisten or not, unless you are being cynical and saying Redis' persistence is hit or mess?

The idea that caching results from SQL queries to disk won't, in general, provide gains is silly. Aggregates, complex joins, high offsets, queries that don't hit indexes and so on can all benefit _greatly_ from having the final results cached. It isn't the network overhead that you are trying to avoid, it's the data transformation.

There are other oddities in there.

Out of curiosity, does PHP code really still use interpolation rather than parameters for queries?

   [select...where] company_id = " . $this->db->escape($company_id) . "";