你可以简单地“添加”数组：

>> $a = array(1, 2, 3);
array (
  0 => 1,
  1 => 2,
  2 => 3,
)
>> $b = array("a" => 1, "b" => 2, "c" => 3)
array (
  'a' => 1,
  'b' => 2,
  'c' => 3,
)
>> $a + $b
array (
  0 => 1,
  1 => 2,
  2 => 3,
  'a' => 1,
  'b' => 2,
  'c' => 3,
)

考虑到你有

$replaced = array('1' => 'value1', '4' => 'value4');
$replacement = array('4' => 'value2', '6' => 'value3');

执行$merge = $replacement + $replaced;将输出：

Array('4' => 'value2', '6' => 'value3', '1' => 'value1');

来自sum的第一个数组将在最终输出中具有值。
执行$merge = $replaced + $replacement;将输出：

Array('1' => 'value1', '4' => 'value4', '6' => 'value3');

我只是想添加另一种可能性，即在保持键的同时进行合并。
除了使用+符号向现有数组添加键/值之外，还可以执行array_replace。

$a = array(
    'foo'  => 'bar',
    'some' => 'string',
    'me'   => 'is original'
);
$b = array(
    42   => 'answer to the life and everything',
    1337 => 'leet',
    'me' => 'is overridden'
);

$merged = array_replace($a, $b);

结果将是：

$merged = array(
    'foo'  => 'bar',
    'some' => 'string',
    'me'   => 'is overridden',
    42     => 'answer to the life and everything',
    1337   => 'leet'
);

相同的键将被后面的数组覆盖。
还有一个array_replace_recursive，它也可以用于子数组。
Live example on 3v4l.org

通过**+**运算符，可以很容易地将两个数组相加或合并，而不需要改变它们的原始索引。这将是非常有帮助的laravel和codeigniter选择下拉列表。

$empty_option = array(
         ''=>'Select Option'
          );

 $option_list = array(
          1=>'Red',
          2=>'White',
          3=>'Green',
         );

  $arr_option = $empty_option + $option_list;

输出将是：

$arr_option = array(
   ''=>'Select Option'
   1=>'Red',
   2=>'White',
   3=>'Green',
 );

OP的要求是保留密钥（保持密钥），而不是重叠（我认为 * 覆盖 *）。在某些情况下，例如数字键，这是可能的，但如果是字符串键，则似乎不可能。
如果你使用array_merge()，数字键总是会重新索引或重新编号。
如果你使用array_replace()，array_replace_recursive()，它将从右到左重叠或覆盖。第一个数组中具有相同键的值将被第二个数组替换。
如果你使用$array1 + $array2作为注解，如果键是相同的，那么它将保留第一个数组的值，但删除第二个数组。

自定义函数。

下面是我的函数，我刚刚写的工作在相同的要求。您可以自由地用于任何目的。

/**
 * Array custom merge. Preserve indexed array key (numbers) but overwrite string key (same as PHP's `array_merge()` function).
 * 
 * If the another array key is string, it will be overwrite the first array.<br>
 * If the another array key is integer, it will be add to first array depend on duplicated key or not. 
 * If it is not duplicate key with the first, the key will be preserve and add to the first array.
 * If it is duplicated then it will be re-index the number append to the first array.
 *
 * @param array $array1 The first array is main array.
 * @param array ...$arrays The another arrays to merge with the first.
 * @return array Return merged array.
 */
function arrayCustomMerge(array $array1, array ...$arrays): array
{
    foreach ($arrays as $additionalArray) {
        foreach ($additionalArray as $key => $item) {
            if (is_string($key)) {
                // if associative array.
                // item on the right will always overwrite on the left.
                $array1[$key] = $item;
            } elseif (is_int($key) && !array_key_exists($key, $array1)) {
                // if key is number. this should be indexed array.
                // and if array 1 is not already has this key.
                // add this array with the key preserved to array 1.
                $array1[$key] = $item;
            } else {
                // if anything else...
                // get all keys from array 1 (numbers only).
                $array1Keys = array_filter(array_keys($array1), 'is_int');
                // next key index = get max array key number + 1.
                $nextKeyIndex = (intval(max($array1Keys)) + 1);
                unset($array1Keys);
                // set array with the next key index.
                $array1[$nextKeyIndex] = $item;
                unset($nextKeyIndex);
            }
        }// endforeach; $additionalArray
        unset($item, $key);
    }// endforeach;
    unset($additionalArray);

    return $array1;
}// arrayCustomMerge

测试。

<?php
$array1 = [
    'cat', 
    'bear', 
    'fruitred' => 'apple',
    3 => 'dog',
    null => 'null',
];
$array2 = [
    1 => 'polar bear',
    20 => 'monkey',
    'fruitred' => 'strawberry',
    'fruityellow' => 'banana',
    null => 'another null',
];

// require `arrayCustomMerge()` function here.

function printDebug($message)
{
    echo '<pre>';
    print_r($message);
    echo '</pre>' . PHP_EOL;
}

echo 'array1: <br>';
printDebug($array1);
echo 'array2: <br>';
printDebug($array2);

echo PHP_EOL . '<hr>' . PHP_EOL . PHP_EOL;

echo 'arrayCustomMerge:<br>';
$merged = arrayCustomMerge($array1, $array2);
printDebug($merged);

assert($merged[0] == 'cat', 'array key 0 should be \'cat\'');
assert($merged[1] == 'bear', 'array key 1 should be \'bear\'');
assert($merged['fruitred'] == 'strawberry', 'array key \'fruitred\' should be \'strawberry\'');
assert($merged[3] == 'dog', 'array key 3 should be \'dog\'');
assert(array_search('another null', $merged) !== false, '\'another null\' should be merged.');
assert(array_search('polar bear', $merged) !== false, '\'polar bear\' should be merged.');
assert($merged[20] == 'monkey', 'array key 20 should be \'monkey\'');
assert($merged['fruityellow'] == 'banana', 'array key \'fruityellow\' should be \'banana\'');

结果。

array1:

Array
(
    [0] => cat
    [1] => bear
    [fruitred] => apple
    [3] => dog
    [] => null
)

array2:

Array
(
    [1] => polar bear
    [20] => monkey
    [fruitred] => strawberry
    [fruityellow] => banana
    [] => another null
)

---
arrayCustomMerge:

Array
(
    [0] => cat
    [1] => bear
    [fruitred] => strawberry
    [3] => dog
    [] => another null
    [4] => polar bear
    [20] => monkey
    [fruityellow] => banana
)

尝试使用array_replace_recursive或array_replace函数

$a = array('userID' => 1, 'username'=> 2);
array (
  userID => 1,
  username => 2
)
$b = array('userID' => 1, 'companyID' => 3);
array (
  'userID' => 1,
  'companyID' => 3
)
$c = array_replace_recursive($a,$b);
array (
  userID => 1,
  username => 2,
  companyID => 3
)

http://php.net/manual/en/function.array-replace-recursive.php