If you loop over $json["abc"][0]
the keys will be a, b, c, d, e and f.
$url = 'http://url.com/json';
$jsondata = file_get_contents($url);
$json = json_decode($jsondata, TRUE);
$data = $json['abc'][0];
foreach ($data as $key => $value) {
var_dump($key, $value);
}
"ba"
would be: $json["abc"][0]["b"][0]["ba"]
"dc"
would be: $json["abc"][0]["d"][1]["dc"]