php decode json array

I have a php page that receives a json object from javascript page, but i was not able to decode the json in php. How to decode the json and store in php such that $arr[0]=[1,2,34,5,2]; $arr[1]=[2,1,34,5,2]; $arr[2]=[8,1,34,5,2]; in php ?

after removing "myString = JSON.stringify(myObject);"
echo $value; outputs "Array"
echo $value[0]; outputs nothing
echo $value->{"key"};  outputs nothing either

how can i actually get the array contents?

javascript:

var mon=[1,2,34,5,2];
var tue=[2,1,34,5,2];
var wed=[8,1,34,5,2];
var myObject = {'key' :'value','key2':'value','key3':'value'};

myObject.key = mon;
myObject.key2 = tue;
myObject.key3 = wed;

 myString = JSON.stringify(myObject); //this line removed

var jsonString = JSON.stringify(myObject);

$.ajax({
    type: "POST",
    url: "n3.php",
    data: {data : jsonString}, 
    cache: false,

    success: function(aaa){ 
        alert("OK");

         $("#pageContent").html(aaa);       
    }
});

php:

<?php
$value = json_decode($_POST['data']);
echo $value;     //this echos the whole json object 
echo $value->{"key"};  //this outputs nothing
?>