Calculate average growth percentage with PHP

I have the following json :

{
  "bpi": {
    "2017-10-31": 21094.2212,
    "2017-11-01": 22052.8615,
    "2017-11-02": 22972.5023,
    "2017-11-03": 23730.9012,
    "2017-11-04": 24483.4171,
    "2017-11-05": 24448.8979,
    "2017-11-06": 22610.3597,
    "2017-11-07": 23292.1128,
    }
}

I needed to get and calculate on average, how many% it increases per day. I do not have any code yet because I have not had the idea how to do this calculation with PHP.