PHP multidimensional array: replace all keys with concatenation of two values

Question

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

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)

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"];
}

EDIT / Underlying problem

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.

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

Answer 1

Ordinarily, you'd be looking for either the array_walk or maybe the array_map function to transform arrays in PHP, but unfortunately neither of them can alter the key of the array that you want to transform. array_walk will preserve the keys, but won't alter them. So sadly, no, there's no built in function to do what you're asking.

Answer 2

Done a few test with the following results (almost all the same).

Test 1:  [0.25861501693726]
Test 2:  [0.20804476737976]
Test 3:  [0.21039199829102]
Oldskool:[0.26545000076294]
Test 4:  [0.35072898864746]

Doing a var_dump() on the merged array will slow things down (as expected), but if you keep it memory the data is not too bad to work with.

And the PHP used to test:

// Construct the raw data
$i = 0;
do {
    $raw[] = array('key1' => mt_rand(10000,99999), 'key2' => mt_rand(10,99), 'count' => $i);
} while(++$i < 100000);

// Test 1
$before = microtime(true);
foreach($raw as $k => $v) {
    $clean[$v['key1'].'_'.$v['key2']] = $v['count'];
}
$after = microtime(true);
echo 'Test 1:['.($after - $before).']<br />';

$clean = false;
$i = 0;

// Test 2
$before = microtime(true);
$max = count($raw);
do {
    $clean[$raw[$i]['key1'].'_'.$raw[$i]['key2']] = $raw[$i]['count'];
} while(++$i < $max);
$after = microtime(true);
echo 'Test 2:['.($after - $before).']<br />';

$clean = false;
$i = 0;

// Test 3
$before = microtime(true);
$max = count($raw);
for($i; $i < $max; $i++) {
    $clean[$raw[$i]['key1'].'_'.$raw[$i]['key2']] = $raw[$i]['count'];
}
$after = microtime(true);
echo 'Test 3:['.($after - $before).']<br />';

$clean = false;

// Test of Oldskool's suggestion
$before = microtime(true);
foreach (array_keys($raw) as $item) {
    $clean[$raw[$item]['key1'].'_'.$raw[$item]['key2']] = $raw[$item]['count'];
}
$after = microtime(true); 
echo 'Test Oldskool:['.($after - $before).']<br />';

$clean = false;
$i = 0;

// Test 4, just for fun
$before = microtime(true);
$max = count($raw);
do {
    $c = array_pop($raw[$i]);
    $clean[join('_', $raw[$i])] = $c;
} while(++$i < $max);
$after = microtime(true);
echo 'Test 4:['.($after - $before).']<br />';

Edit: Added a test for Oldskool example.