Call-time pass-by-reference

when i click on Private message settings in posting panel of ACP at the top of the page appear this message:"
Warning: Call-time pass-by-reference has been deprecated - argument passed by value; If you would like to pass it by reference, modify the declaration of utf8_normalize_nfc(). If you would like to enable call-time pass-by-reference, you can set allow_call_time_pass_reference to true in your INI file. However, future versions may not support this any longer. in C:\web\www\phpBB3\includes\acp\acp_board.php on line 352"