解析JSON并使用PHP循环它

My problem is loading a JSON file into my php page. With XML / RSS I don't have problems and also in javascript I succeeded, but for this school project I need to load JSON data in PHP.

{"tvshows":[
      {
         "showname":"Parenthood ",
         "date":"2014-12-28",
         "fullTitle":"Parenthood (2010) 2x10 \"Happy Thanksgiving \"",
         "img":"http:\/\/zapp.trakt.us\/images\/episodes\/117-2-10.jpg?4"
      },
      {
         "showname":"Parenthood ",
         "date":"2014-12-28",
         "fullTitle":"Parenthood (2010) 2x11 \"Damage Control\"",
         "img":"http:\/\/zapp.trakt.us\/images\/episodes\/117-2-11.jpg?4"
      },
      {
         "showname":"Benched ",
         "date":"2014-12-27",
         "fullTitle":"Benched 1x09 \"A New Development\"",
         "img":"http:\/\/zapp.trakt.us\/images\/episodes\/33221-1-9.jpg?3"
      }
   ]
}

When I use this php code doesn't see an output:

$string = file_get_contents("url");
$json = json_decode($string, true);

print_r($json);

After that I want to walk through the tv shows and show the showname values. How should I do that?

foreach($json["tvshows"] as $key=>$val){
    echo $key."-".$val["showname"]."<br/>";
}