get pure string from json string like ‘ufe0fJ’ with php

here is my json string
{ "opa": "O_ufe0fPufe0f_ufe0fAufe0f" }

when i decode my json string
object(stdClass)#16 (1) { ["opa"]=> string(25) "O_ufe0fPufe0f_ufe0fAufe0f" }

i want to get without this ->ufe0f symbols
result should be “O_P_A”

P.S. it can be any other character like [udbff, udc00, etc.]