mysqli_report() is a nice function for improving your queries and code during development and testing phase. On the basis of the flags it reports errors from mysqli function calls or queries which don't use an index (or use a bad index).

The flags are :
MYSQLI_REPORT_OFF Turns reporting off
MYSQLI_REPORT_ERROR Report errors from mysqli function calls
MYSQLI_REPORT_STRICT Report warnings from mysqli function calls
MYSQLI_REPORT_INDEX Report if no index or bad index was used in a query
MYSQLI_REPORT_ALL Set all options (report all)

bool mysqli_report ( int $flags )