Pass by reference (C++ only) Pass-by-reference means to pass the reference of an argument in the calling function to the corresponding formal parameter of the called function. ざっくりというと「参照渡しは変数しか使えませんよ〜」っていうエラーです。. Example: [php] $file_name = "image01.jpg"; echo end(explode(‘.’, $file_name)); [/php] This renders the above notice.