php基础有关问题二维数组求和

php 基础问题二维数组求和 ！
数组：
$a = array(array("a"=>"aa","a1"=>1,"a2"=>1),array("a"=>"bb","a1"=>2,"a2"=>2),array("a"=>"cc","a1"=>3,"a2"=>3),array("a"=>"aa","a1"=>1,"a2"=>1),array("a"=>"bb","a1"=>5,"a2"=>5))
第一个键值相同的元素 求和
结果
$r = array(array("a"=>"aa","a1"=>2,"a2"=>2),array("a"=>"bb","a1"=>7,"a2"=>7),array("a"=>"cc","a1"=>3,"a2"=>3));
------解决方案--------------------

$a = array(
  array("a" => "aa", "a1" => 1, "a2" => 1),
  array("a" => "bb", "a1" => 2, "a2" => 2),
  array("a" => "cc", "a1" => 3, "a2" => 3),
  array("a" => "aa", "a1" => 1, "a2" => 1),
  array("a" => "bb", "a1" => 5, "a2" => 5),
);
$res = array();
foreach($a as $r) {
  if(! isset($res[$r['a']])) $res[$r['a']] = $r;
  else {
    $res[$r['a']]['a1'] += $r['a1'];
    $res[$r['a']]['a2'] += $r['a2'];
  }
}
print_r(array_values($res));

Array
(
    [0] => Array
        (
            [a] => aa
            [a1] => 2
            [a2] => 2
        )

    [1] => Array
        (
            [a] => bb
            [a1] => 7
            [a2] => 7
        )

    [2] => Array
        (
            [a] => cc
            [a1] => 3
            [a2] => 3
        )

)