I've just changed the PHP version from PHP 5.3 through 5.6 to PHP 7.x in the IDE (Run & Debug -> Settings and Code Insight ->Settings) but when I start typing anything that is derived from mysqli code completion isn't working. I get a message saying that code completion is not available - Failed to find "mysqli" class (or trait). If I switch back to PHP 5.3 through 5.6 code completion works. The PHP script works fine (connects to the database and runs SQL) for both versions of PHP so something is finding the mysqli code.

it works fine for me

It you are still having problem, update IDE to the latest version and give parser time to re-parse all standard definitions.