Unable saving an associative array using add_post_meta() function - php

Question

I need to run an encryption function on each member of an associative array prior to it being saved to wp_postmeta table. My form allows dynamic add/delete of rows. I've been trying to get this to work using array_walk().
This is how the form is configured:
<input type="text" id="z_my_data[][username]" name="z_my_data[0][username]">
<input type="text" id="z_my_data[][password]" name="z_my_data[0][password]">
This is ran on the 'save_post` action:
// Save encrypted data to post meta
if (isset($_POST['z_my_data'])) {
// Get posted form variables
$my_data = $_POST['z_my_data'];
// Encrypt each member of each row
for ($i = 0; $i < count($my_data); $i++) {
$cryptKey = $this->cryptKey;
array_walk($my_data[strval($i)], create_function('&$val', 'global $cryptKey; $val = Crypto::encrypt($val, $cryptKey);'));
}
if ( ! add_post_meta( $post_id, '_my_data', $my_data, true ) ) {
update_post_meta( $post_id, '_my_data', $my_data);
}
} else {
delete_post_meta( $post_id, '_my_data' );
}
I can see that the data is being encrypted. Here is a print_r($my_data) after the for loop:
Array
(
[0] => Array
(
[username] => ®ØåÛâÏ0…"ë°?mˤÙ
[password] => xSFç„L¶·3z˜'J0ÖRÅÎj
)
)
But the post meta key is not created and no error is generated. The meta key doesn't exist in the postmeta table, yet add_post_meta() returns false and the key/value is never added.
Does anyone see what I'm doing wrong?
I'm not sure how to tell, but I think the [0] is a named key and not an index key. I say that because I can create more than one and delete the [0] element and the single element remaining still shows [1] using print_r.

Answer 1

In your if ( ! add_post_meta( $post_id, '_my_data', $my_data, true ) ) { you are using add_post_meta() function and last argument is set to true.
It should be false instead, because you are inserting an array and NOT a string.
For this reason your if statement is not working properly.
Instead, your code should be (as add_post_meta last argument default value is false):
if ( ! add_post_meta( $post_id, '_my_data', $my_data ) ) {
update_post_meta( $post_id, '_my_data', $my_data );
}
Alternatively, you could also use this:
if ( ( !empty( get_post_meta( $post_id, '_my_data' ) ) ) {
update_post_meta( $post_id, '_my_data', $my_data );
}
References:
WordPress Code Reference - add_post_meta
WordPress Code Reference - update_post_meta
WordPress Code Reference - get_post_meta