PHP 連想配列で重複キーを処理するにはどうすればよいですか?

function array_push_associative(&$arr) {
    $args = func_get_args();
    foreach ($args as $arg) {
        if (is_array($arg)) {
            foreach ($arg as $key => $value) {
                $arr[$key] = $value;
                $ret++;
            }
        } else {
            $arr[$arg] = "";
        }
    }
    return $ret;
}

$arr = [];
array_push_associative($arr, ['42' => 56], ['42' => 86], ['42' => 97]);

$arr = ['42' => 97];

$arr = [
    '42' => [56, 86, 97],
    '51' => [64, 52],
];