Why the key-value-pair won't be added?

I'm trying to add a new key-value-pair at the end of every users element:

<?php
$json = '[
  {
    "date": "2014-10-09T17:38:19Z",
    "users": [
      {
        "name": "Peter",
        "age": 20
      },
      {
        "name": "Anne",
        "age": 25
      },
      {
        "name": "William",
        "age": 30
      }
    ]
  }
]';

addData ( $json );
function addData($json) {
    $obj = json_decode ( $json, true );
    foreach ( $obj as $items ) {
        foreach ( $items ['users'] as $users ) {
            $array = array (
                    "myKey" => "myValue" 
            );
            array_push ( $users, $array );
        }
    }
    $json = json_encode ( $obj );

    echo $json;
}
?>

So the new json should look like

[  
   {  
      "date":"2014-10-09T17:38:19Z",
      "users":[  
         {  
            "name":"Peter",
            "age":20,
            "myKey":"myValue"
         },
         {  
            "name":"Anne",
            "age":25,
            "myKey":"myValue"
         },
         {  
            "name":"William",
            "age":30,
            "myKey":"myValue"
         }
      ]
   }
]

Instead, I get my old json as an output, without the new key-value-pair.

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