Question

The following code will add the categories selector widget to the WordPress Page editor interface…

add_action('admin_menu', 'my_post_categories_meta_box');
function my_post_categories_meta_box() {
    add_meta_box('categorydiv', __('Categories'), 'post_categories_meta_box', 'page', 'side', 'core');
}

What I would like to do is to figure out how to modify the resulting category listing so that it only contains a predefined list of hard coded categories that I define.

Since I’m adding this via my custom theme, it will only appear on the page editor when my theme is active on the site. And I have some specific “handler” categories that my theme installs into the site and later uses to determine layout elements. I only want these specific categories to be listed in this particular instance of the categories widget.

Answer 1

Use a modified version of post_categories_meta_box, in which you change the call to wp_category_checklist to a modified version, wp_category_checklist_modified.

post_categories_meta_box_modified:

function post_categories_meta_box_modified($post) {
?>
<ul id="category-tabs">
    <li class="ui-tabs-selected"><a href="#categories-all" tabindex="3"><?php _e( 'All Categories' ); ?></a></li>
    <li class="hide-if-no-js"><a href="#categories-pop" tabindex="3"><?php _e( 'Most Used' ); ?></a></li>
</ul>

<div id="categories-pop" class="ui-tabs-panel" style="display: none;">
    <ul id="categorychecklist-pop" class="categorychecklist form-no-clear" >
        <?php $popular_ids = wp_popular_terms_checklist('category'); ?>
    </ul>
</div>

<div id="categories-all" class="ui-tabs-panel">
    <ul id="categorychecklist" class="list:category categorychecklist form-no-clear">
        <?php wp_category_checklist($post->ID, false, array($cat1_id, $cat2_id.... ,$catn_id), $popular_ids) ?>
    </ul>
</div>

<?php if ( current_user_can('manage_categories') ) : ?>
<div id="category-adder" class="wp-hidden-children">
    <h4><a id="category-add-toggle" href="#category-add" class="hide-if-no-js" tabindex="3"><?php _e( '+ Add New Category' ); ?></a></h4>
    <p id="category-add" class="wp-hidden-child">
        <label class="hidden" for="newcat"><?php _e( 'Add New Category' ); ?></label><input type="text" name="newcat" id="newcat" class="form-required form-input-tip" value="<?php _e( 'New category name' ); ?>" tabindex="3" aria-required="true"/>
        <label class="hidden" for="newcat_parent"><?php _e('Parent category'); ?>:</label><?php wp_dropdown_categories( array( 'hide_empty' => 0, 'name' => 'newcat_parent', 'orderby' => 'name', 'hierarchical' => 1, 'show_option_none' => __('Parent category'), 'tab_index' => 3 ) ); ?>
        <input type="button" id="category-add-sumbit" class="add:categorychecklist:category-add button" value="<?php _e( 'Add' ); ?>" tabindex="3" />
        <?php wp_nonce_field( 'add-category', '_ajax_nonce', false ); ?>
        <span id="category-ajax-response"></span>
    </p>
</div>
<?php
endif;

}

I’ve only change the line of the original function

<?php wp_category_checklist($post->ID, false, false, $popular_ids) ?>

to

<?php wp_category_checklist_modified($post->ID, false, false, $popular_ids) ?>

wp_category_checklist_modified:

function wp_category_checklist_modified( $post_id = 0, $descendants_and_self = 0, $selected_cats = false, $popular_cats = false, $include_cats = array() ) {
    $walker = new Walker_Category_Checklist;
    $descendants_and_self = (int) $descendants_and_self;
    $cat_ids_list = implode(',', $include_cats);

    $args = array();

    if ( is_array( $selected_cats ) )
        $args['selected_cats'] = $selected_cats;
    elseif ( $post_id )
        $args['selected_cats'] = wp_get_post_categories($post_id);
    else
        $args['selected_cats'] = array();

    if ( is_array( $popular_cats ) )
        $args['popular_cats'] = $popular_cats;
    else
        $args['popular_cats'] = get_terms( 'category', array( 'fields' => 'ids', 'orderby' => 'count', 'order' => 'DESC', 'number' => 10, 'hierarchical' => false ) );

    if ( $descendants_and_self ) {
        $categories = get_categories( "child_of=$descendants_and_self&hierarchical=0&hide_empty=0&include=$cat_ids_list" );
        $self = get_category( $descendants_and_self );
        array_unshift( $categories, $self );
    } else {
        $categories = get_categories('get=all&include='. $cat_ids_list);
    }

    // Post process $categories rather than adding an exclude to the get_terms() query to keep the query the same across all posts (for any query cache)
    $checked_categories = array();
    for ( $i = 0; isset($categories[$i]); $i++ ) {
        if ( in_array($categories[$i]->term_id, $args['selected_cats']) ) {
            $checked_categories[] = $categories[$i];
            unset($categories[$i]);
        }
    }

    // Put checked cats on top
    echo call_user_func_array(array(&$walker, 'walk'), array($checked_categories, 0, $args));
    // Then the rest of them
    echo call_user_func_array(array(&$walker, 'walk'), array($categories, 0, $args));
}

Here I’ve added an extra argument to wp_category_checklist_modified, $include_cats, in which you can specify the categories ids, then I use this list in the two calls to get_categories passing it as the include parameter.

These functions are not documented (as much as I am be able to find), so I’ve had to take a look to the source code.

Then you will simply use

function my_post_categories_meta_box() {
   add_meta_box('categorydiv', __('Categories'), 'post_categories_meta_box_modified', 'page', 'side', 'core');
}

Hope this helps.