How foreach loop work to sum value of same key in associative array

I have associative array.Operation of the below code is that it will sum all the array index’s value which key is similar, but i did not understand how it operated.

function add_array_vals($arr) {
  $sums = [];
  foreach ( $arr as $key => $val ) {
    $key = strtoupper($key);
    if ( !isset($sums[$key]) ) {
      $sums[$key] = 0;
    }
    $sums[$key] = ( $sums[$key] + $val );
  }
  return $sums;
}

$array = ['KEY' => 5, 'TEST' => 3, 'Test' => 10, 'Key'=> 2];
$sums = add_array_vals($array);
var_dump($sums);

//Outputs
// KEY => int(7)
// TEST => int(13)

i have problem in two portion of above code one is:

if ( !isset($sums[$key]) ) { $sums[$key] = 0; }

another is:

$sums[$key] = ( $sums[$key] + $val );

In this portion,how it identify the same key of array to sum them because keys position is random.

It will be really helpful if anyone clarify it.

Answer

if ( !isset($sums[$key]) ) { $sums[$key] = 0; }

$sums starts as an empty array, but we’re trying to add numbers to it. If we try to add a number to $sums[$key] that isn’t initialized yet, we’ll get an error, so we initialize it by setting it at zero the first time we encounter it.

$sums[$key] = ( $sums[$key] + $val );

This is the second part of the previous line. $sums[$key] is at this point either zero, thanks to the previous line, or an integer, because we’ve encountered it before in the loop.

The $key in $sums[$key] is going to be either ‘TEST’ or ‘KEY’ in this code.