<?php
$json = '{
"Autenticacao": {
"login": "123",
"senha": "123"
}
}';
if(!isset($obj->Autenticacao->login)) { }
echo $obj->Autenticacao->login;
PD9waHAKJGpzb24gPSAnewogICAgIkF1dGVudGljYWNhbyI6IHsKICAgICAgICAibG9naW4iOiAiMTIzIiwKICAgICAgICAic2VuaGEiOiAiMTIzIgogICAgfQp9JzsKCiRvYmogPSBqc29uX2RlY29kZSgkanNvbik7CmlmKCFpc3NldCgkb2JqLT5BdXRlbnRpY2FjYW8tPmxvZ2luKSkgewoJZGllKCdlcnJvciBicnJyaGgnKTsKfQplY2hvICRvYmotPkF1dGVudGljYWNhby0+bG9naW47