好了，问题来了，这是保存数据的完整代码。下面对这些问题进行了解释：

/**
 * Adds a box to the main column on the Post and Page edit screens.
 */

function nss_mood_add_meta_box() {
    //$id, $title, $callback, $post_type, $context,$priority, $callback_args 
    add_meta_box(\'nss_mood_id\',\'Set your mood\',\'nss_mood_cb\',\'post\');
} 

add_action(\'add_meta_boxes\',\'nss_mood_add_meta_box\');

// Dispalying form and taking input
function nss_mood_cb() {
    global $post;
    echo \'<input type="hidden" name="mood_meta_box_nonce" value="\'.wp_create_nonce(basename(__FILE__)).\'" />\';
    $mood_value = get_post_meta( $post->ID, \'mood_value_key\', true );

    // Creating our form
    echo \'<p>\';
        echo \'<label for="mood_value_key">What is your mood today ? </label></br></br>\';
        echo \'<input type="text" class="widefat" name="mood_value_key" id="mood_value_key" value="\' . esc_attr( $mood_value ) . \'" />\';
    echo \'</p>\';
}

// Checking value of the form and updating
function save_nss_mood_data($post_id) {

    /*
     * We need to verify this came from our screen and with proper authorization,
     * because the save_post action can be triggered at other times.
     */

    // Check if our nonce is set.
    if ( empty( $_POST[\'mood_meta_box_nonce\'] ) || !wp_verify_nonce( $_POST[\'mood_meta_box_nonce\'], basename(__FILE__)) ) return;

    // If this is an autosave, our form has not been submitted, so we don\'t want to do anything.
    if ( defined( \'DOING_AUTOSAVE\' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Check the user\'s permissions.
    if ( isset( $_POST[\'post_type\'] ) ) {

        if ( ! current_user_can( \'edit_post\', $post_id ) ) {
            return;
        }

    }

    /* OK, it\'s safe for us to save the data now. */

    // Make sure that it is set.
    if ( ! isset( $_POST[\'mood_value_key\'] ) ) {
        return;
    }

    // Sanitize user input.
    $mood_data = sanitize_text_field( $_POST[\'mood_value_key\'] );

    // Update the meta field in the database.
    update_post_meta( $post_id, \'mood_value_key\', $mood_data );
}

add_action(\'save_post\',\'save_nss_mood_data\');

您的功能nss_mood_cb() 需要$post->ID 但是没有$post 那里所以你需要$post 变量，我使用global $post.