PDO is PHP's database-agnostic interface; the DSN string (sqlite::memory: here) tells it which driver to use. Setting ERRMODE_EXCEPTION is essential: without it, PDO silently swallows query errors and you have to check return values everywhere. prepare parses and validates the SQL once; execute runs it with new parameter values, which means repeated inserts share the parsed plan. Named placeholders (:name, :email) are easier to read than positional ? and impervious to argument-order mistakes.

Use fetch() in a while loop for streaming large result sets; the rows come from the driver one at a time so memory stays bounded. fetchAll() materialises everything at once and is fine for small queries. The fetch mode controls the row shape: FETCH_ASSOC returns [col => value] (most common for JSON output), FETCH_OBJ returns stdClass objects, FETCH_NUM returns positional arrays. Pass parameters in the execute([...]) form rather than concatenating into the SQL; the database treats the values as data, not code, which is what closes the SQL injection vector.

Wrap related writes in beginTransaction / commit so a partial failure rolls the whole batch back via rollBack. The try / catch (Throwable) shape matters: if any prepared statement throws (the ERRMODE_EXCEPTION flag we set earlier), control jumps to the catch block and rollBack() undoes whatever was already executed in the transaction. Without a transaction, a failed second insert would leave the first insert visible to other connections, breaking invariants that depend on both writes succeeding. Always pair beginTransaction with both a commit on the success path AND a rollback in the catch block; orphaned transactions hold locks and break replication.