thx for reply
use join can modify the sign_msg
but how to write it back in the middle of the array?

4__taka__email__url__icon__time__message
3__12monkey__email__url__icon__time__message
2__sarah__email__url__icon__time__message
1__john__email__url__icon__time__message
(after i modify third message , how to write into the orginal place ?)