Update value of a associative array with update_post_meta

Question

I am trying to change the value of a specific key of an associative array which was saved in my post meta. This is how my array is saved in a custom field:

Array ( 
    [0] => Array (
        [321] => Zedity is an innovative content Editor to create posts or pages
    )
    [1] => Array (
        [310] => A shortcode that includes other posts
    )
)

This is how I am trying to update the value of key 321:

global $post;
$postid = $post->ID;
$add_to_ID = $_POST['wpd_plugin_note_id'];
$note = $_POST['wpd_plugin_note'];

$existing_list = get_post_meta($postid, 'my_list_items', TRUE );

foreach ($existing_list as $key => $value) {
    $existing_list[$key][$add_to_ID] = $note;
}    
update_post_meta($postid,'my_list_items',$existing_list);

It works, but after the update the array looks like this, instead of updating the value of key 321, the key=value pair is being added 2 times:

Array ( 
    [0] => Array (
        [321] => my new value for 321
    ) 
    [1] => Array (
        [310] => A shortcode that includes other posts
        [321] => my new value for 321
    )
)

Can anyone tell me what I am doing wrong? Thanks

Answer 1

This is plain PHP (logic):

foreach ($existing_list as $key => $value)
    if (isset($existing_list[$key][$add_to_ID]))
        $existing_list[$key][$add_to_ID] = $note;

What you did before was not update the sub-element with key 321 for each element but set the sub-element with that particular key for each element—which is in essence: update where already defined, define otherwise.