Can't Get Metabox Data Saved Assistance Needed

Question

I'm trying to create meta box on a post create/edit page. I faced the problem:

1. When I create a post from scratch the data from my meta box is saved as expected
2. When I try to update an existing post with a new meta box data the data is not saved.

I experimented with a couple of action hooks which I got from wordpress docs and none of them seem to work. Please check the code below:

<?
    add_action('save_post','callback');       // works when a post is created from scratch
    add_action('update_post','callback');     // doesn't work
    add_action('pre_post_update','callback'); // doesn't work
    add_action('publish_post','callback');    // doesn't work

    function callback( $post_ID ) {
        echo "THE POST IS SAVED";

        // Code for saving metabox here
        ...
    };

Don't see any reason why this thing is not working as expected. If anyone faced the same issues please help! I'm desperate

Have a nice day!

Answer 1

The save_post hook is the only one you need, also, your callback function is missing a parameter, check this example:

/**
 * Save post metadata when a post is saved.
 *
 * @param int $post_id The ID of the post.
 */
function save_book_meta( $post_id ) {

    /*
     * In production code, $slug should be set only once in the plugin,
     * preferably as a class property, rather than in each function that needs it.
     */
    $slug = 'book';

    // If this isn't a 'book' post, don't update it.
    if ( $slug != $_POST['post_type'] ) {
        return;
    }

    // - Update the post's metadata.

    if ( isset( $_REQUEST['book_author'] ) ) {
        update_post_meta( $post_id, 'book_author', sanitize_text_field( $_REQUEST['book_author'] ) );
    }

    if ( isset( $_REQUEST['publisher'] ) ) {
        update_post_meta( $post_id, 'publisher', sanitize_text_field( $_REQUEST['publisher'] ) );
    }

    // Checkboxes are present if checked, absent if not.
    if ( isset( $_REQUEST['inprint'] ) ) {
        update_post_meta( $post_id, 'inprint', TRUE );
    } else {
        update_post_meta( $post_id, 'inprint', FALSE );
    }
}
add_action( 'save_post', 'save_book_meta' );

The example can be found in the Codex page for the save_post action hook.