PHP: array merge while preserving numeric keys

array_merge_preserve.php

<?php

/* 
Eg. $array1:
array:1 [
    3 => array:1 [
      "subscription_id" => 27
    ]
  ]
  
$array2: 
array:1 [
    1 => array:1 [
      "subscription_id" => 28
    ]
  ]

array_merge_preserve($array1, $array2);

Expected output:
 3 => array:1 [
    "subscription_id" => 27
  ]
 1 => array:1 [
    "subscription_id" => 28
  ]

*/

function array_merge_preserve(...$arrays)
{
    $acc = [];
    foreach ($arrays as $index => $array){
        if (empty($acc))
            $acc = $array;
        else
            $acc += $array;
    }
    return $acc;
}