According to the documentation, a prepared query provides a significant performance benefit if you're running a query multiple times because the overhead of the MySQL server parsing the query only happens once. I'm wondering what exactly they mean by "multiple times" there.
I.e., say you have a web page that runs a query one time. Now say that page gets called 50 times per second. Does it make more sense from a performance standpoint to prepare() the query (thereby requiring two roundtrips to the DB server; one to prepare the query, one to run it) or to just send the query normally (which only requires one roundtrip)? Is MySQL and/or the PHP mysqli driver smart enough to realize when a query was prepare()'d in a previous invocation?
No. PHP is a "shared nothing" architecture, so every resource associated with one request (one page view) is discarded at the end of that request. Prepared queries are not available to a subsequent database connections.
The scenario in which you would get benefit from a prepared query is when you prepare it and execute it many times during the same PHP request.