Some projects call for unconventional practices. Adding categories or tags to WordPress pages is probably one of the practices.

<pre>// Add Page tags and categories to WordPress Pages
function add_category_box_on_page(){
    //add meta box
    add_meta_box('categorydiv', __('Categories'), 'post_categories_meta_box', 'page', 'side', 'low');
    add_meta_box('tagsdiv-post_tag', __('Page Tags'), 'post_tags_meta_box', 'page', 'side', 'low');
    add_action('save_post', 'pp_page_cats_tags_save_postdata');
}
 
add_action('admin_menu', 'add_category_box_on_page');
 
function pp_page_cats_tags_save_postdata( $post_id ) {
 
    $pp_post_type = $_POST['post_type'];
 
    // verify if this is an auto save routine. If it is our form has not been submitted, so we dont want
    // to do anything
    if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ){
 
    }else{
        // Check permissions
        if ( 'page' == $pp_post_type ) {
            if ( current_user_can( 'edit_page', $post_id ) ){
 
                $pp_post_cats = array();
 
                foreach($_REQUEST['post_category'] as $key=>$val){
                    $pp_post_cats[] = $val;
                }
                wp_set_post_categories( $post_id, $pp_post_cats );
            }
        }
    }
}</pre>

Add this code as well and you will ensure that pages are also displayed in WordPress archives and post lists:

<pre>// This filter ensures that the custom added Page tags and categories are displayed in WordPress archives
add_filter( 'pre_get_posts' , 'ucc_include_custom_post_types' );
function ucc_include_custom_post_types( $query ) {
	global $wp_query;
	 
	/* Don't break admin or preview pages. */
	if ( !is_preview() && !is_admin() && !is_page() && !is_single() && !is_home() ) {
		$args = array(
		'public' => true ,
		'_builtin' => false
		);
		$output = 'names';
		$operator = 'and';
		 
		$post_types = get_post_types( $args , $output , $operator );
		$post_types = array_merge( $post_types , array( 'post', 'page' ) );
	 
		if ($query->is_feed) {
			// Do feed processing here.
		} else {
			$my_post_type = get_query_var( 'post_type' );
			if ( empty( $my_post_type ) )
			$query->set( 'post_type' , $post_types );
		}
	}
	 
	return $query;
}</pre>

Line 5 is kind of important. Here you can use Conditional Tags to prevent pages from being added under those conditions. For instance, we’ve added !is_home() because although we may want pages in our archive pages, we didn’t want them in our main blog index. Have fun.

This code was taken from The WP Ninjas.