PHP多维数组组合

我希望能找到如何获得这样的结构的帮助：

[pad] => Array
    (
        [padi] => Array
            (
                [Date] => 2016-01-01
                [Val] => 1
                [Premium] => 100
            )
        [padi] => Array
            (
                [Date] => 2016-04-01
                [Val] => 1
                [Premium] => 200
            )
        [padi] => Array
            (
                [Date] => 2016-07-01
                [Val] => 1
                [Premium] => 300
            )
        [padi] => Array
            (
                [Date] => 2016-11-01
                [Val] => 1
                [Premium] => 400
            )
    )

我有4个变量：

$p1 = array('padi' =>
                    array('Date' => $Date_0, 'Val' => $Val, 'padiPremium' => $padiPremium_0));
$p2 = array('padi' =>
                    array('Date' => $Date_1, 'Val' => $Val, 'padiPremium' => $padiPremium_1));
$p3 = array('padi' =>
                    array('Date' => $Date_1, 'Val' => $Val, 'padiPremium' => $padiPremium_1));
$p4 = array('padi' =>
                    array('Date' => $Date_1, 'Val' => $Val, 'padiPremium' => $padiPremium_1));

我已经尝试过array_push、array_merge和array_combine，但这对我来说根本不起作用，或者只是我的方法错了。如果有任何建议，我将不胜感激！