mysql_free_result() will free
all memory associated with the result identifier result.
mysql_free_result() only needs
to be called if you are concerned about how much memory is
being used for queries that return large result sets. All
associated result memory is automatically freed at the end of
the script's execution.
成功回傳TRUE失敗回傳FALSE。
For downward compatibility
mysql_freeresult() can also be used. This is deprecated,
however.