Leer el valor clave JSON PHP
// json object.
$contents = '{"firstName":"John", "lastName":"Doe"}';
// Option 1: through the use of an array.
$jsonArray = json_decode($contents,true);
$key = "firstName";
$firstName = $jsonArray[$key];
// Option 2: through the use of an object.
$jsonObj = json_decode($contents);
$firstName = $jsonObj->$key;
Ivan The Terrible