dodawanie elementu do jsona

function testJsonCode(&$json)
{
        foreach ($treeJson as $tj) {
            $tj['geeks'] = "baz"; //ten json nie dodaje sie do tablicy json
        }
        $treeJson['geeks'] = "baz"; //ten json dodaje sie do tablicy json
        
}

^ array: [▼
  0 => array:2 [▼
    "id" => 19
    "test" => array:6 [▶]
  ]
  1 => array:2 [▶]
  "geeks" => "baz"

foreach ($treeJson as &$tj) {
  /* ... */
}
unset($tj);