первое совпадение из JSON

есть переменная

PHP:

1. $json='{"1":"off","2":"on","3":"off","4":"off","5":"off","6":"off","7":"off","8":"off","9":"off","10":"off","11":"off","12":"off","13":"off","14":"off","15":"off","16":"off","17":"off","18":"off","19":"on","20":"on","21":"on","22":"off","23":"off","24":"off","25":"off","26":"off","27":"off","28":"off","29":"off","30":"off","31":"off","32":"off","33":"on","34":"on","35":"on","36":"on","37":"off"}';

как из нее вытащить первое совпадение со значение с значением "on"

т.е на данном примере

HTML:

1. "2":"on",

мне нужно в переменную $one присвоить значение 2

PHP:

1. $one ="2";

 

спасибо!