How to get values from json encode in php?

I have a json like this

{
  "data": [
    {
      "description": "~~ funny ~~

for more pictures , visit www.1mpics.com

Para mais imagens , visite www.1mpics.com",
      "media": {
        "image": {
          "height": 429,
          "src": "https://scontent.xx.fbcdn.net/hphotos-xfa1/v/t1.0-9/281318_366934083403548_1944634610_n.jpg?oh=6d6321885ff621cc79c404b0aa559c21&oe=5617A0FB",
          "width": 550
        }
      },
      "target": {
        "id": "366934083403548",
        "url": "https://www.facebook.com/1mpics/photos/a.348356355261321.82963.237513286345629/366934083403548/?type=1"
      },
      "title": "Timeline Photos",
      "type": "photo",
      "url": "https://www.facebook.com/1mpics/photos/a.348356355261321.82963.237513286345629/366934083403548/?type=1"
    }
  ]
}

now I want "src" from the json. Therefore I create code like this where I pass the json array through post method

$json = $_POST["a"];
$json = json_decode($json, true);
echo $json['scr'];

but it is not working.