Metabox doesn’t retain values

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Many of the comments below refer to code used for debugging, which for clarity I’ve removed.

I should have spotted this soon – you are adding only one row into the postmeta table, and that row has the key _my_meta. Before inserting the array int the table, WordPress serializes the array. When you retrieve the meta, you are retrieving a serialized array which you would then need to unserialize.

However, I don’t think this is what you want, (nor is it advisable). It would be better to save each of the values in the array as a separate key-value pair in the database (this would be more effecient, and allowing querying of specific key-values etc).

To do that you need to loop through the received data array and add each key-value individually: (I have relaced your code with appropriate comments)

function my_meta_save($post_id) {

// verify this is not an auto save routine. 
if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) return $post_id;

//Perform nonce checks & permissions (for pages and posts)

//Sanitize the input 

//Declare a white list of 'keys' with their defaults (or null)
//- i.e. this code will not add any other keys:
$whitelist = array(
    'title'=>null,
    'img01-title'=>null,
    'img01-url'=>null
);
   //It would probably make sense to do this before sanitzing. 
    $new_data = shortcode_atts($whitelist,$new_data);

   //Now add/update or remove key-values appropriately
   foreach ($whitelist as $key=>$value){
        //If the key has a value, update it. If it doesn't already exist it will add it
        if(!is_null($whiltelist[$key]))
              update_post_meta($post_id,$key,$value);
        else
              delete_post_meta($post_id,$key);
    }

    //No need to return, this is an action not a filter
 }

Remarks

  1. I’ve added an auto-save check so that data is updated when the post/page auto saves
  2. You’ve checked permissions if the post is a page – you should do this for posts too.
  3. I’ve made use of shortcode_atts which will remove any keys that you haven’t declared in $whitelist. It will assign default values to keys that are missing/don’t have a value.
  4. update_post_meta will add a new key if it doesn’t exist