PHP:配列の添字を振り直さず配列同士をマージする

tl;dr

array_replace()、array_replace_recursive()を使う。

$array1 = [
    'hoge' => 'fuga',
    'piyo',
    'fizz',
    'arr' => [
        'key' => 'val'
    ],
    'aaa' => 'bbb',
    'arr2' => [
        'key1' => 'val1',
        'key2' => 'val2',
        'str1'
    ],
    20 => 30,
];
$array2 = [
    'hoge' => 'hogehoge',
    'piyopiyo',
    'foo' => 'bar',
    'buzz',
    'arr' => 'str',
    'arr2' => [
        'key2' => 'value2',
        'key3' => 'value3',
        'str2'
    ],
    21 => 40,
];
print_r(array_replace($array1, $array2));
print_r(array_replace_recursive($array1, $array2));

array_replace

Array
(
    [hoge] => hogehoge
    [0] => piyopiyo
    [1] => buzz
    [arr] => str
    [aaa] => bbb
    [arr2] => Array
        (
            [key2] => value2
            [key3] => value3
            [0] => str2
        )

    [20] => 30
    [foo] => bar
    [21] => 40
)

array_replace_recursive

Array
(
    [hoge] => hogehoge
    [0] => piyopiyo
    [1] => buzz
    [arr] => str
    [aaa] => bbb
    [arr2] => Array
        (
            [key1] => val1
            [key2] => value2
            [0] => str2
            [key3] => value3
        )

    [20] => 30
    [foo] => bar
    [21] => 40
)

他のマージ関数の場合

すでに他所で書かれておりますが、以下にまとめてみました。
※同じ配列で実行した結果です。

array_merge

print_r(array_merge($array1, $array2));

result

Array
(
    [hoge] => hogehoge
    [0] => piyo
    [1] => fizz
    [arr] => str
    [aaa] => bbb
    [arr2] => Array
        (
            [key2] => value2
            [key3] => value3
            [0] => str2
        )

    [2] => 30
    [3] => piyopiyo
    [foo] => bar
    [4] => buzz
    [5] => 40
)

array_merge_recursive

print_r(array_merge_recursive($array1, $array2));

result

Array
(
    [hoge] => Array
        (
            [0] => fuga
            [1] => hogehoge
        )

    [0] => piyo
    [1] => fizz
    [arr] => Array
        (
            [key] => val
            [0] => str
        )

    [aaa] => bbb
    [arr2] => Array
        (
            [key1] => val1
            [key2] => Array
                (
                    [0] => val2
                    [1] => value2
                )

            [0] => str1
            [key3] => value3
            [1] => str2
        )

    [2] => 30
    [3] => piyopiyo
    [foo] => bar
    [4] => buzz
    [5] => 40
)

array1 + array2

print_r($array1 + $array2);

result

Array
(
    [hoge] => fuga
    [0] => piyo
    [1] => fizz
    [arr] => Array
        (
            [key] => val
        )

    [aaa] => bbb
    [arr2] => Array
        (
            [key1] => val1
            [key2] => val2
            [0] => str1
        )

    [20] => 30
    [foo] => bar
    [21] => 40
)

array2 + array1

print_r($array2 + $array1);

result

Array
(
    [hoge] => hogehoge
    [0] => piyopiyo
    [foo] => bar
    [1] => buzz
    [arr] => str
    [arr2] => Array
        (
            [key2] => value2
            [key3] => value3
            [0] => str2
        )

    [21] => 40
    [aaa] => bbb
    [20] => 30
)

ついで

array_replace、array_replace_recursiveの使用できるバージョンは PHP 5 >= 5.3.0 となります。
そんな方いらっしゃらないかと思いますが、もし、5.3未満で使いたい場合は、以下に代替関数を載せておきます。
なお、テスト通してないので、ご使用には十分ご注意下さい。

if (!function_exists('array_replace')) {
    function array_replace(array $a1, array $a2) {
        foreach ($a2 as $k => $v) {
            $a1[$k] = $v;
        }
        return $a1;
    }
}
if (!function_exists('array_replace_recursive')) {
    function array_replace_recursive(array $a1, array $a2) {
        foreach ($a2 as $k => $v) {
            if (is_array($v) && isset($a1[$k]) && is_array($a1[$k])) {
                $v  = array_replace_recursive($a1[$k], $v);
            }
            $a1[$k] = $v;
        }
        return $a1;
    }
}