Expand PHP Array one level before array_merge

I have a simple array like this:

    Array(
      [Key1] => value1
      [Key2] => value2
      [Key3] => value3
    )

which I merge with a similar array (array_merge) and gets this.
(If the key already exist only the value will be updated)

    Array(
      [Key1] => value1
      [Key2] => value2
      [Key3] => value3
      [Key4] => value4
      [Key5] => value5
    )

But I want to add a date to when each item was either added or updated. Like this:

    Array(
       [0] => Array(
          [Key1] => Value1
          [Date] => 2023-07-10 02:14
       )
       [1] => Array(
          [Key2] => Value2
          [Date] => 2023-06-25 05:30
       )
       [2] => Array(
          [Key3] => Value3
          [Date] => 2023-07-12 16:40
       )
       [3] => Array(
          [Key4] => Value4
          [Date] => 2023-07-13 09:51
       )
       [4] => Array(
          [Key5] => Value5
          [Date] => 2023-07-13 09:51
       )
)

So the new data is a one level array at first, I need to add a the current date/time (Format not important right now) to each key/level and then merge it with the old array. The old array will always be stored as a two level array.

I hope I make any sense here

You can do this by looping over both arrays individually. This should work regardless of how many arrays you have in total.

$arrayOne = [
	'Key1' => 'value1',
	'Key2' => 'value2'
];

$arrayTwo = [
	'Key2' => 'value2'
];

foreach([$arrayOne, $arrayTwo] as $array)
{
	foreach($array as $key => $value)
	{
		$output[$key] = [
			$key => $value,
			'Date' => date('Y-m-d H:i:s.u')
		];
	}
	
	sleep(1); // Just for testing purposes.
}

Since this runs almost instantly, I had to add sleep(1) to show that it works. The second value is the only value that is overwritten and thus shows the updated timestamp.

array(2) {
    ["Key1"] => array(2) {
        ["Key1"] => string(6) "value1"
        ["Date"] => string(26) "2023-07-13 08:08:49.000000"
    }
    ["Key2"]=> array(2) {
        ["Key2"] => string(6) "value2"
        ["Date"]=> string(26) "2023-07-13 08:08:50.000000"
    }
}