Sum multidimensional associative array values while preserving key names - php

Question

There are many nice Q&A on Stackoverflow on how to take a multidimensional associative array and sum values in it. Unfortunately, I can't find one where the key names aren't lost.
For example:
$arr=array(
array('id'=>'1', 'amount'=>'5'),
array('id'=>'1', 'amount'=>'5'),
array('id'=>'2', 'amount'=>'1'),
array('id'=>'2', 'amount'=>'3')
);
I want the resulting array to look like this:
$result=array(
array('id'=>'1', 'amount'=>'10'),
array('id'=>'2', 'amount'=>'4')
);
Unfortunately the only thing I can figure out how to do is this:
$result = array();
foreach($arr as $amount){
if(!array_key_exists($amount['id'], $arr))
$result[$amount['id']] =0;
$result[$amount['id']] += $amount['amount'];
}
Which when echo'd as follows produces (notice the lack of the keys' words "id" and "amount"):
foreach($result as $id => $amount){
echo $id."==>".$amount."\n";
}
1==>10
2==>4

Answer 1

This is just to show that you already had the data you originally needed, though, the answer you accepted is a better way to deal with it.
You have the following to start with right?
$arr = array(
array('id'=>'1', 'amount'=>'5'),
array('id'=>'1', 'amount'=>'5'),
array('id'=>'2', 'amount'=>'1'),
array('id'=>'2', 'amount'=>'3')
);
Output
Array
(
[0] => Array
(
[id] => 1
[amount] => 5
)
[1] => Array
(
[id] => 1
[amount] => 5
)
[2] => Array
(
[id] => 2
[amount] => 1
)
[3] => Array
(
[id] => 2
[amount] => 3
)
)
Then you run it through the following algorithm:
$summedArr = array();
foreach ($arr as $amount) {
$summedArr['amount'][$amount['id']] += $amount['amount'];
$summedArr['id'][$amount['id']] = $amount['id'];
}
Now, disregarding the Notice warnings that are produced since you are referencing indexes that don't yet exist, this outputs the following:
Output
Array
(
[amount] => Array
(
[1] => 10
[2] => 4
)
[id] => Array
(
[1] => 1
[2] => 2
)
)
Do you see the keys, yet? Because I do.
Now iterate over the array:
foreach ($summedArr as $key => $value) {
echo $k . "==>" . $v . "\n";
}
Output
amount==>Array
id==>Array
That's not what you want, though. You want:
foreach ($summedArr as $key => $arr) {
foreach ($arr as $v) {
echo $key . "==>" . $v;
}
}

Answer 2

Why not use that method, and then reconstruct the array you want at the end?
$results = array();
foreach ($arr as $id=>$amount) {
$results[] = array('id' => $id, 'amount' => $amount);
}
Any other way of doing this would be more computationally expensive.