Description:
------------
When you convert array from stdClass you will get array with many stdClasses.
Isset on index of array element will not work correctly if you try it.

Test script:
---------------
$id = '11400';
$json = '{"11400":{"test":"1","test":"2","xyz":"abc"}}';
// yes, I know second parametr, but it should work too
$decodedJson = (array)json_decode($json);
if (isset($decodedJson[$id])) {
    echo 'exists!';
} else {
    echo 'not exists!';
}