This occur since last beta update.
Matomo version: 3.13.1-b2
PHP Fatal error: Call to undefined method mysqli_stmt::get_result() in (...) piwik/core/Tracker/Db/Mysqli.php
Tracking is completely down : all request are throwing 500 errors.
This has been solved by installing php-mysqlnd : https://dev.mysql.com/downloads/connector/php-mysqlnd/
Seeing in https://www.php.net/manual/en/mysqli-stmt.get-result.php this is indeed only available with my mysqlnd. We'll need to change that.