三元操作返回“未定义属性”

Question

I have switched from using this ternary operation to if/elseif/else, but I would really like to know why this doesn't work. I keep getting the error:

ErrorException [ Notice ]: Undefined property: stdClass::$error

If I were to switch the order of the conditions so that the $res->response piece was 3rd, then I get the same error but for that property instead. It makes no sense to me, and testing it on Codepad.org (which uses PHP 5.2) it works as expected: http://codepad.org/gwteijIe

Here's the test code in question:

$output = '{"error":{"message":"This is a test error"}}';
$res = json_decode($output);

$error = isset($res->response) ? $res->response->message :
     isset($res->error) ? $res->error :
     isset($res->error->message) ? $res->error->message :
     $output;

echo $error;

(The reason why it checks for $res->error and $res->error->message is because the API we're using will return any one of the three conditions we're checking for. Disclaimer: it's not my API!)

Any insight as to why I am not getting the results I expect? Many thanks in advance for the help.

Edit:

To clarify, this is what I am trying to achieve:

if (isset($res->response))
{
    $error = $res->response->message;
}
elseif (isset($res->error->message))
{
    $error = $res->error->message;
}
elseif (isset($res->error))
{
    $error = $res->error;
}

Answer 1

Don't nest ternary operator.

You see the error because of the order in which ternary operators are evaluated; it would be far better and more maintainable to just write it out like this:

if (isset($res->response)) {
  $error = $res->response->message;
} elseif (isset($res->error)) {
  if (isset($res->error->message)) {
    $error = $res->error->message;
  } else {
    $error = $res->error;
  }
} else {
  $error = $output;
}

Your old code evaluates like this; even then it's non-obvious what really happens:

((isset($res->response) ? $res->response->message :
isset($res->error)) ? $res->error :
isset($res->error->message)) ? $res->error->message :
$output;

See also: Comparison Operators