Why does not my metabox save?

Question

I know there are many questions about variants of this, but I have not been able to find an answer.

I have a metabox on the post type page, containing nothing but a checkbox. It seems like it won't save no matter what I do. Here is all the code for the metabox.

/*--------------------------------------------------------------------------*
 * Register metabox
/*--------------------------------------------------------------------------*/
function kasparabi_page_left_menu() {
    add_meta_box( 'kasparabi-left-menu-meta', __( 'Left Menu', 'kasparabi' ), 'kasparabi_render_left_menu_meta_box', 'page', 'side' );
}
add_action( 'add_meta_boxes', 'kasparabi_page_left_menu' );

/*--------------------------------------------------------------------------*
 * Callbacks
/*--------------------------------------------------------------------------*/
function kasparabi_render_left_menu_meta_box($post) {
    wp_nonce_field( basename( __FILE__ ), 'kasparabi-left-menu-meta_nonce' );

    ?>
        <p>
            <div>
                <label for="left-menu-checkbox">
                    <input type="checkbox" name="left-menu-checkbox" <?php (get_post_meta( $post->ID, 'left_menu_checkbox', true) == 'on') ? ' checked="checked"' : ''; ?> />
                    <?php _e( 'Display left menu', 'kasparabi' )?>
                </label>
            </div>
        </p>
    <?php
}

/*--------------------------------------------------------------------------*
 * Save functions
/*--------------------------------------------------------------------------*/
function kasparibi_left_menu_meta_save( $post_id, $post ) {

    // Checks save status
    $is_autosave = wp_is_post_autosave( $post_id );
    $is_revision = wp_is_post_revision( $post_id );
    $is_valid_nonce = ( isset( $_POST[ 'kasparabi-left-menu-meta_nonce' ] ) && wp_verify_nonce( $_POST[ 'kasparabi-left-menu-meta_nonce' ], basename( __FILE__ ) ) ) ? 'true' : 'false';

    // Exits script depending on save status
    if ( $is_autosave || $is_revision || !$is_valid_nonce ) {
        return $post_id;
    }

    /* Get the post type object. */
    $post_type = get_post_type_object( $post->post_type );

    /* Check if the current user has permission to edit the post. */
    if ( !current_user_can( $post_type->cap->edit_post, $post_id ) )
        return $post_id;

    /* Get the posted data and sanitize it for use as an HTML class. */
    $new_meta_value = ( isset( $_POST['left-menu-checkbox'] ) ? sanitize_html_class( $_POST['left-menu-checkbox'] ) : '' );

    $meta_key = 'left_menu_checkbox';
    $meta_value = get_post_meta( $post->ID, $meta_key, true);

    /* If a new meta value was added and there was no previous value, add it. */
    if ( $new_meta_value && '' == $meta_value )
        add_post_meta( $post_id, $meta_key, $new_meta_value, true);

    /* If the new meta value does not match the old value, update it. */
    elseif ( $new_meta_value && $new_meta_value != $meta_value )
        update_post_meta( $post_id, $meta_key, $new_meta_value );

    /* If there is no new meta value but an old value exists, delete it. */
    elseif ( '' == $new_meta_value && $meta_value )
        delete_post_meta( $post_id, $meta_key, $meta_value );
}
add_action( 'save_post', 'kasparibi_left_menu_meta_save' );

Answer 1

I notice a couple of things.

First, your callback uses two parameters but you don't ask for the second parameter when you hook it in. This:

add_action( 'save_post', 'kasparibi_left_menu_meta_save' );

Should be:

add_action( 'save_post', 'kasparibi_left_menu_meta_save' ,1 ,2 );

But, on the other hand, I don't see where you use that parameter at all. Maybe I missed it. You could probably just leave that second parameter out of the callback.

The real problem is here though:

<?php (get_post_meta( $post->ID, 'left_menu_checkbox', true) == 'on') ? ' checked="checked"' : ''; ?>

You don't echo the checked attribute.

<?php echo (get_post_meta( $post->ID, 'left_menu_checkbox', true) == 'on') ? ' checked="checked"' : ''; ?>

In other words, the value does save, but your code does not display the checked status correctly.

You could simplify that line with WordPress's checked function though.

<?php checked(get_post_meta( $post->ID, 'left_menu_checkbox', true),'on',true) ?>