将关联对添加到PHP数组

我有一个数组,我试着写

array_push($json['Request']['Header'], "key" => "val");

但我收到一个错误.写下面的工作,但它添加一个数组而不仅仅是键/ val

array_push($json['Request']['Header'], array("key" => "val"));

..
[0] => Array
        (
            [key] => val
        )

//i would like
...
[key] => val

解决方法:

为什么不简单写：

$json['Request']['Header'] = array();
$json['Request']['Header']['key'] = 'val';