<?php
$json = '{
"Autenticacao": [{
"login": "123",
"senha": "123"
}]
}';
if(!isset($obj->Autenticacao->login)) { }
echo $obj->Autenticacao->login;
PD9waHAKJGpzb24gPSAnewogICAgIkF1dGVudGljYWNhbyI6IFt7CiAgICAgICAgImxvZ2luIjogIjEyMyIsCiAgICAgICAgInNlbmhhIjogIjEyMyIKICAgIH1dCn0nOwoKJG9iaiA9IGpzb25fZGVjb2RlKCRqc29uKTsKaWYoIWlzc2V0KCRvYmotPkF1dGVudGljYWNhby0+bG9naW4pKSB7CglkaWUoJ2Vycm9yIGJycnJoaCcpOwp9CmVjaG8gJG9iai0+QXV0ZW50aWNhY2FvLT5sb2dpbjs=