Error: Commands out of sync; you can't run this command now

Basically, your flush doesn’t account for mysqli‘s tendency to throw an “out of sync” error when the previous query did not read all its results.

This can be reproduced by a simple CALL ... query, followed by any query. This would result in:

Error: Commands out of sync; you can’t run this command now.

Please add the following code to your flush method:

while ( $this->dbh->more_results() ) $this->dbh->next_result();

This will prevent unread results from desynchronizing mysqli. See https://www.php.net/manual/en/mysqli.query.php#102904 and other comments about the sync error.

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