Ph two-dimensional array sorting problem, with detailed requirements

The two-dimensional array sorting problem of ph. for help, the two-dimensional array with detailed requirements for initial status is like this $ a & nbsp; array (& nbsp; array (name & gt; a, num & gt; 1, parentNum & gt; 0), & nbsp; array (name & gt; B, num & gt; 2. parent ph: two-dimensional array sorting problem, request help, with detailed requirements included
The two-dimensional array of the initial state is like this.

$a = array(
  array('name'=>'a','num'=>'1','parentNum'=>'0'),
  array('name'=>'b','num'=>'2','parentNum'=>'0'),
  array('name'=>'c','num'=>'3','parentNum'=>'0'),
  array('name'=>'d','num'=>'4','parentNum'=>'1'),
  array('name'=>'e','num'=>'5','parentNum'=>'2'),
  array('name'=>'f','num'=>'6','parentNum'=>'0'),
  array('name'=>'g','num'=>'7','parentNum'=>'0'),
)

Sort by name by default, but change now
It is still sorted by the name field, but also by the parentNum field. parentNum indicates its parent category, so the final result should be

  array('name'=>'a','num'=>'1','parentNum'=>'0'),
    array('name'=>'d','num'=>'4','parentNum'=>'1'),
  array('name'=>'b','num'=>'2','parentNum'=>'0'),
   array('name'=>'e','num'=>'5','parentNum'=>'2'),
  array('name'=>'c','num'=>'3','parentNum'=>'0'),
  array('name'=>'f','num'=>'6','parentNum'=>'0'),
  array('name'=>'g','num'=>'7','parentNum'=>'0'),

How can I implement it using code? I tried for a long time. why not?
------ Solution ----------------------

$a = array(
  array('name'=>'a','num'=>'1','parentNum'=>'0'),
  array('name'=>'b','num'=>'2','parentNum'=>'0'),
  array('name'=>'c','num'=>'3','parentNum'=>'0'),
  array('name'=>'d','num'=>'4','parentNum'=>'1'),
  array('name'=>'e','num'=>'5','parentNum'=>'2'),
  array('name'=>'f','num'=>'6','parentNum'=>'0'),
  array('name'=>'g','num'=>'7','parentNum'=>'0'),
);

foreach($a as $r) {
  $p[] = $r['parentNum'] ? $r['parentNum'] : $r['num'];
}
array_multisort($p, $a);
print_r($a);

Array
(
    [0] => Array
        (
            [name] => a
            [num] => 1
            [parentNum] => 0
        )

    [1] => Array
        (
            [name] => d
            [num] => 4
            [parentNum] => 1
        )

    [2] => Array
        (
            [name] => b
            [num] => 2
            [parentNum] => 0
        )

    [3] => Array
        (
            [name] => e
            [num] => 5
            [parentNum] => 2
        )

    [4] => Array
        (
            [name] => c
            [num] => 3
            [parentNum] => 0
        )

    [5] => Array
        (
            [name] => f
            [num] => 6
            [parentNum] => 0
        )

    [6] => Array
        (
            [name] => g
            [num] => 7
            [parentNum] => 0
        )

)