If the query raises an error, returns FALSE. If it is successful and
there is a (possibly empty) result set (such as with a SELECT query),
returns a result identifier. If the query was successful and there were
no results, returns TRUE.
Note:
In PHP 5.0.0 and up, this function returns the number of rows affected by
the query (if > 0 and applicable to the statement type). A query that
succeeded, but did not affect any rows (e.g. an UPDATE of a non-existent
record) will return TRUE.