How to merge some fields of second array into first array after comparing the values of keys common in both arrays?

Question

My question sounds bit confusing but actually it's simple. Let me describe you the scenario first. I've two arrays viz. $grid_data and $sent_data. Both the arrays generate dynamically depending on the user input. So for instance I'm printing below the contents of both of these arrays:

$grid_data = Array
    (
        [0] => Array
            (
                [newsletter_id] => 1
                [newsletter_name] => Eywa Solutions
                [newsletter_subject] => Holi Wishes
                [newsletter_email_body] => Happy Holi to all the friends
                [newsletter_call_to_action_status] => 0
                [newsletter_call_to_action_text] => 
                [newsletter_call_to_action_link] => 
                [newsletter_status] => 2
                [newsletter_schedule] => 0
                [newsletter_created_date] => Mar 17 2014, 16:21 pm
                [newsletter_updated_date] => 0
            )

        [1] => Array
            (
                [newsletter_id] => 2
                [newsletter_name] => Akshay Nikte
                [newsletter_subject] => The Don
                [newsletter_email_body] => How are yoy Nikte Saheb?
                [newsletter_call_to_action_status] => 0
                [newsletter_call_to_action_text] => 
                [newsletter_call_to_action_link] => 
                [newsletter_status] => 2
                [newsletter_schedule] => 0
                [newsletter_created_date] => Mar 18 2014, 06:52 am
                [newsletter_updated_date] => 0
            )

    )

$sent_data = Array
        (
            [0] => Array
                (
                    [newsletter_id] => 1
                    [newsletter_sent_count] => 5
                    [newsletter_sent_date] => 1395121193
                )

            [1] => Array
                (
                    [newsletter_id] => 2
                    [newsletter_sent_count] => 7
                    [newsletter_sent_date] => 1395121227
                )

        )

Now what I want to achieve is compare the values of key [newsletter_id] from both of the above arrays with each other. If match is found append the key-value pairs([newsletter_sent_count], [newsletter_sent_date]) to the respective array from $grid_data array. If match is not found then insert blank values for those keys([newsletter_sent_count], [newsletter_sent_date]). In above case the finally generated $grid _data should look like below:

$grid_data = Array
    (
        [0] => Array
            (
                [newsletter_id] => 1
                [newsletter_name] => Eywa Solutions
                [newsletter_subject] => Holi Wishes
                [newsletter_email_body] => Happy Holi to all the friends
                [newsletter_call_to_action_status] => 0
                [newsletter_call_to_action_text] => 
                [newsletter_call_to_action_link] => 
                [newsletter_status] => 2
                [newsletter_schedule] => 0
                [newsletter_created_date] => Mar 17 2014, 16:21 pm
                [newsletter_updated_date] => 0
                [newsletter_sent_count] => 5
                [newsletter_sent_date] => 1395121193
            )

        [1] => Array
            (
                [newsletter_id] => 2
                [newsletter_name] => Akshay Nikte
                [newsletter_subject] => The Don
                [newsletter_email_body] => How are yoy Nikte Saheb?
                [newsletter_call_to_action_status] => 0
                [newsletter_call_to_action_text] => 
                [newsletter_call_to_action_link] => 
                [newsletter_status] => 2
                [newsletter_schedule] => 0
                [newsletter_created_date] => Mar 18 2014, 06:52 am
                [newsletter_updated_date] => 0
                [newsletter_sent_count] => 7
                [newsletter_sent_date] => 1395121227
            )

    )

I want to do this in an most optimum way. I tried using array_merge() to achieve this but I was not succeed in my attempt. So can anyone from the pool of talent help me in this regard please? Thanks in advance. If you need any further information I can provide you the same. The attempt made by me is as follows:

$arr_length = count($grid_data);


for($i=0; $i < $arr_length; $i++) {
  $track =0;
  foreach($sent_data as $value) {
    if($value['newsletter_id'] === $grid_data[$i]['newsletter_id'] ) {
      array_merge($grid_data[$i], $value);
      $track=1;
    }  
  }
  if($track === 0) {
    //insert blank value here
  }
}

Answer 1

First convert $sent_data into an associative array:

$sent_assoc = array();
foreach ($sent_data as $el) {
    $sent_assoc[$el['newsletter_id']] = $el;
}

Then do the merging:

$default_sent = array('newsletter_sent_count' => null, 'newsletter_sent_date' => null);
foreach ($grid_data as &$el) {
    $id = $el['newsletter_id'];
    $sent = isset($sent_assoc[$id]) ? $sent_assoc[$id] : $default_sent;
    $el = array_merge($el, $sent);
}

The reference &$el is used so that the assignment updates the original array.

Without the reference the second loop looks like:

foreach ($grid_data as $i => $el) {
    $id = $el['newsletter_id'];
    $sent = isset($sent_assoc[$id]) ? $sent_assoc[$id] : $default_sent;
    $grid_data[$i] = array_merge($el, $sent);
}