PHP多维数组：用两个值的串联替换所有键

我在PHP中有一个多维数组，其中外部数组包含数千个项目，其中每个项目都是一个数组本身，其值分别为 key1， key2和 count：

I have a multidimensional array in PHP, where the outer array contains several thousands items and each item inside is an array itself with the values "key1", "key2" and "count":

 myExistingArray (size=99999 VERY BIG)
      public 0 => 
        array (size=3)
          'key1' => string '15504' 
          'key2' => string '20'
          'count' => string '1'
      public 1 => 
        array (size=3)
          'key1' => string '15508' (length=5)
          'key2' => string '20' (length=2)
          'count' => string '2' (length=1)
      public 2 => 
        array (size=3)
          'key1' => string '15510' (length=5)
          'key2' => string '20' (length=2)
          'count' => string '5' (length=1)
....many more similar items

我想将其转换为一个非常简单的数组，其中 key1和 key中的前一个值被连接为一个新的键，该键指向相应的 count值，如下所示：

I want to transform this into a very simple array, where the former values from "key1" and "key" are concatenated to be a new key that points to the corressponding "count" value like so:

  myNewArray (size=99999 VERY BIG)
      <key1>_<key2> => <count>
      15504_20 => string '1' (length=1)
      15508_20 => string '2' (length=1)
      15510_20 => string '5' (length=1)

性能对我来说很重要，因为外部数组有多个一千个项目。 PHP中有快速方法吗？我唯一得到的只是一个简单的迭代，但这对我来说似乎很慢：

Performance is very important for me since the outer array has several thousand items. Is there a fast method in PHP? The only thing I got was a simple iteration, but this seems to slow for me:

// works but I am looking for a faster version
$myNewArray = array();
foreach ($myExistingArray as $item) {
  $myNewArray [$item["key1"]."_".$item["key1"]]=$item["count"];
}

编辑/基本问题

有些人正确地补充说，我当前的解决方案已经在O（n）中，并提到PHP中没有内置函数可以加快此过程。

Some people rightfully added that my current solution is already in O(n) and mentioned that there is no built-in function in PHP to speed this up.

我从mysql数据库查询中获得 myExistingArray。我基本上有工作对象，并想按其状态和event_id将它们分组。查询与此类似：

I get "myExistingArray" from a mysql database query. I basically have job objects and want to group them by their status and their event_id. The query similiar to this:

select count(job.id) as count, job.status as key1, job.event_id as key2
from job
group by job.status, job.event_id

I希望重新排列键，以便以后可以轻松访问具有特定状态的特定事件的作业计数。

I want to rearrange the keys so that later I can easily access the count of jobs for a certain event with a certain status.