I’m passing some data through AJAX to a function in my functions.php, and am trying to save that as metadata to post ‘101’ with a key name “test1”. My PHP function is:

function fztest(){

    $datum = $_POST['datum'];
    $post_ID = $_POST['post_ID'];

    add_post_meta($post_ID, 'test1', $datum);

    // And since $datum and $post_ID are correctly returning the values I want, but the meta is not saving, 
    // I've tried troubleshooting by putting some manual operations in various places below:

    add_post_meta(101, 'test2', 'foobar');

    wp_die();

    add_post_meta(101, 'test3', 'foobar');

}
add_action('wp_ajax_foobar', 'fztest');
add_action('wp_ajax_nopriv_foobar', 'fztest');

add_post_meta(101, 'test4', 'foobar');

But when I try to return get_post_meta(101, ‘test1’), for example – only ‘test4’ above works – it just returns an empty string for test1, test2 and test3.

My JS is:

$.ajax({
    url: fz.ajax_url,
    type: "POST",
    data: {
        action: "foobar",
        datum: "foobar",
        post_ID: fz.post_ID,
    },
    success: function(response){

    }
})

… and that is correctly passing the values through to the PHP function, and logging a response when I have tried one.

I can’t see why add_post_meta would not work in the PHP function? There’s a similar question here where the solution indicates it clearly should and CAN work. How do I update_post_meta() or add_post_meta() with an AJAX call. And their code looks the same as mine as far as I can see.

Can anyone see what I’m doing wrong?