合并二维数组的有关问题

合并二维数组的问题
Array ( [108-3752685-0517825] => Array ( [0] => 6300-00 ) [107-5010391-7170621] => Array ( [0] => 0054-00 ) [105-5621773-8281824] => Array ( [0] => 0049-00 ) )

Array ( [2200320000123] => Array ( [0] => 0054-00 ) [2200320123501] => Array ( [0] => 0049-00 ) [220032012395200] => Array ( [0] => 0049-00 ) )

Array ( [108-3752685-0517825] => Array ( [0] => 6300-00 ) [107-5010391-7170621] => Array ( [0] => 0054-00 ) [105-5621773-8281824] => Array ( [0] => 0049-00 ) [0] => Array ( [0] => 0054-00 ) [1] => Array ( [0] => 0049-00 ) [2] => Array ( [0] => 0049-00 ) )

用array_merge合并后，第二个数组的键名没有了，是我用法不对？？
------解决思路----------------------
不是这样的吗？

$a = array (
  '108-3752685-0517825' => 
  array (
    0 => '6300-00',
  ),
  '107-5010391-7170621' => 
  array (
    0 => '0054-00',
  ),
  '105-5621773-8281824' => 
  array (
    0 => '0049-00',
  ),
);
$b = array (
  '2200320000123' => 
  array (
    0 => '0054-00',
  ),
  '2200320123501' => 
  array (
    0 => '0049-00',
  ),
  '220032012395200' => 
  array (
    0 => '0049-00',
  ),
);
$t = array_merge($a, $b);
print_r($t);

Array
(
    [108-3752685-0517825] => Array
        (
            [0] => 6300-00
        )

    [107-5010391-7170621] => Array
        (
            [0] => 0054-00
        )

    [105-5621773-8281824] => Array
        (
            [0] => 0049-00
        )

    [2200320000123] => Array
        (
            [0] => 0054-00
        )

    [2200320123501] => Array
        (
            [0] => 0049-00
        )

    [220032012395200] => Array
        (
            [0] => 0049-00
        )

)