如何合并到索引数组？

$array1 = array('a', 'b', 'c');
$array2 = array('d', 'e', 'f');
$result = array_merge($array1, $array2);
print_r($result);

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
    [4] => e
    [5] => f
)

$array1 = array('a', 'b', 'c');
$array2 = array('d', 'e', 'f');
$result = [...$array1, ...$array2];
print_r($result);

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
    [4] => e
    [5] => f
)

$array1 = array('a', 'b', 'c');
$array2 = array('d', 'e', 'f');
$result = $array1;
foreach ($array2 as $value) {
    $result[] = $value;
}
print_r($result);

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
    [4] => e
    [5] => f
)