stmt_init() seems to clear previous (possibly erroneous) results on the DB connection, which means you don't necessarily need to use it but it could make the code more robust.In a PHPUnit test, I had a sequence of prepared queries on the same connection. One of them fetched a row from a SELECT but didn't keep fetching until it drained the connection, so it left some stale results. When the next query did this:<?php$db = $this->getConnection()->getDbConnection();$preparedQuery = $db->prepare ($query);?>the prepare() call generated an error: "Could not prepare query: Commands out of sync; you can't run this command now." Changing to this:<?php$db = $this->getConnection()->getDbConnection();$preparedQuery = $db->stmt_init();$preparedQuery->prepare ($query);?>resolved the problem.

you can use $stmt = $mysqli->prepare(); directly without stmt-init() . i think there is no need for stmt-init .