Page navigation shortcode for WordPress

page-navigation.php

<?php
/**
 * @package Page Navigation Shortcode
 * @version 0.1
 */
/*
Plugin Name: Page Navigation Shortcode
Description: This plugin automatically adds links to the previous and next pages at the bottom of page content. User's may disable for select pages as well on the page edit screen.
Author: TLT Studio
Version: 0.1
Author URI: http://studio.tlt.psu.edu/
*/


if(!class_exists('Page_Navigation')){

class Page_Navigation {

	const CACHE_GROUP = 'page-navigation';

	function __construct(){
		add_filter('the_content', array(&$this,'add_shortcode'));
		add_shortcode('page_navigation', array(&$this,'do_shortcode'));
		
		if(is_admin()){
			add_action('save_post_page', array(&$this,'purge_cache'));
			add_action('save_post_page', array(&$this,'save_meta'));
			add_action('add_meta_boxes', array(&$this,'add_meta_box'));
		}
	}

	function add_meta_box(){
		add_meta_box(
        	'page-navigation-meta-box', // id, used as the html id att
        	__( 'Page Navigation' ), // meta box title, like "Page Attributes"
        	array(&$this,'meta_box'), // callback function, spits out the content
        	'page', // post type or page. We'll add this to pages only
        	'side', // context (where on the screen
        	'low' // priority, where should this go in the context?
    	);
	}

	function meta_box($post){
		wp_nonce_field( 'page_navigation_meta_box', 'page_navigation_meta_box_nonce' );

		$value = get_post_meta( $post->ID, '_page_navigation_append', true );
		if($value == ''){
			$value = 1;
		}
		echo '<input type="checkbox" id="page-navigation-append" name="page_navigation_append" value="1" '. checked($value, 1,false) .' />';
		echo '<label for="page-navigation-append">';
		_e( 'Show Page Navigation');
		echo '</label> ';
		echo '<p>If checked links to the next and previous pages will be shown at the bottom of the page content.</p>';
	}

	function save_meta($post_id ){
		if ( ! isset( $_POST['page_navigation_meta_box_nonce'] ) ) {
			return;
		}
		if ( ! wp_verify_nonce( $_POST['page_navigation_meta_box_nonce'], 'page_navigation_meta_box' ) ) {
			return;
		}
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return;
		}
		if ( ! current_user_can( 'edit_page', $post_id ) ) {
			return;
		}

		$value = (isset($_POST['page_navigation_append'])) ? '1' : '0';
		update_post_meta($post_id, '_page_navigation_append', $value);
	}

	function add_shortcode($content){
		global $post;
		// only do this for pages
		if($post->post_type != 'page')
			return $content;

		// don't add if the shortcode is already present
		if(strpos($content, '[page_navigation') !== false)
			return $content;

		$value = get_post_meta($post->ID, '_page_navigation_append',true);
		if($value == '0')
			return $content;

		// value is not present or set already - so go ahead and add the shortcode
		return "{$content}\n\n[page_navigation]";
	}

	function do_shortcode($args){
		global $post;
		if($post->post_type != 'page')
			return;

		// not used yet - but hope to support 
		// querying the post tree via the an actual menu
		$args = shortcode_atts(array(
			'navigation' => 'pages'
		), $args);

		// we have a namespace key for the cache so we can invalidate it be incrementing the prefix when pages are updated
		$prefix = wp_cache_get(self::CACHE_GROUP.'_key', self::CACHE_GROUP);
		if ($prefix === false ){
			$prefix = 1;
			wp_cache_set(self::CACHE_GROUP.'_key', $prefix , self::CACHE_GROUP);
		}

		$cache_key = "p{$prefix}-{$post->ID}";
		$output = wp_cache_get($cache_key, self::CACHE_GROUP);
		
		if($output === false){
			$next = $this->get_next_page($args['navigation']);	
			$prev = $this->get_previous_page($args['navigation']);
			$output = "{$next}{$prev}";
			wp_cache_set($cache_key, $output, self::CACHE_GROUP);
		} 
		
		return $output;
	}


	function get_next_page($navigation = 'pages'){
		global $post;

		$get_pages = get_pages('child_of='.$post->post_parent.'&sort_column=menu_order&sort_order=asc');
		$pages_count = count($get_pages);
		
		// look for child page first
		for($p=0; $p < $pages_count; $p++) {
	  		// get the array key for our entry
			if ($post->ID == $get_pages[$p]->ID) break;
		}
		// assign our next key
		$next_key = $p+1;

		$output = false;
		// if there isn't a value assigned for the previous key, go all the way to the end
		if (isset($get_pages[$next_key])) {
			$anchor_name = $get_pages[$next_key]->post_title;
			$output = '<a class="page-navigation next" style="text-align:right; float:right; display:block; width:49%;" href="'.get_permalink($get_pages[$next_key]->ID).'" title="'.esc_attr($anchor_name).'">Next Page: '.esc_html($anchor_name).'</a>';
		} else {
			$get_parent_pages = get_pages('sort_column=menu_order&sort_order=asc');
			$parent_page_count = count($get_parent_pages);
	
			for($pp=0; $pp < $parent_page_count; $pp++) {
	  			// get the array key for our entry
				if ($post->ID == $get_parent_pages[$pp]->ID) break;
			}
			// assign our next key
			$parent_page_count = $pp+1;
			if (isset($get_parent_pages[$parent_page_count])) { 
				$anchor_name = $get_parent_pages[$parent_page_count]->post_title;
				$output = '<a class="page-navigation next"  style="text-align:right; float:right; display:block; width:49%;" href="'.get_permalink($get_parent_pages[$parent_page_count]->ID).'" title="'.esc_attr( $anchor_name ) .'">Next Page: '.esc_html($anchor_name).'</a>';
			}
		}

		return $output;
	}

	function get_previous_page($navigation = 'pages'){
		global $post;

		$get_pages = get_pages('child_of='.$post->post_parent.'&sort_column=menu_order&sort_order=asc');
		$pages_count = count($get_pages);
		
		// look for child page first
		for($p=0; $p < $pages_count; $p++) {
	  		// get the array key for our entry
			if ($post->ID == $get_pages[$p]->ID) break;
		}
		// assign our next key
		$prev_key = $p-1;

		$output = false;
		// if there isn't a value assigned for the previous key, go all the way to the end
		if (isset($get_pages[$prev_key])) {
			$anchor_name = $get_pages[$prev_key]->post_title;
			$output = '<a style="float:left; display:block; width:49%;"  class="page-navigation prev" href="'.get_permalink($get_pages[$prev_key]->ID).'" title="'.esc_attr($anchor_name).'">Previous Page: '.esc_html($anchor_name).'</a>';
		} else if ($post->post_parent != 0) {
			$anchor_name = get_the_title($post->post_parent);		
			$output = '<a style="float:left; display:block; width:49%;" class="page-navigation prev" href="'.get_permalink($post->post_parent).'" title="'.esc_attr( $anchor_name ).'">Previous Page: '.esc_html($anchor_name).'</a>';
		}
		
		return $output;
	}


	function purge_cache(){
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return;
		}
		// increment our prefix value when a page is updated, this will invalidate
		// previous set cache items, let memcached garbage collection take care of the
		// removing the bad values from memory
		wp_cache_increment(self::CACHE_GROUP.'_key', 1, self::CACHE_GROUP);
	} 


}


$page_navigation = new Page_Navigation();

}