Snippets Shortcode plugin. Add code snippets as a Custom Post Type. Display in regular posts and pages using a Shortcode. Rendered with Google Prettify in frontend.

code-snippets-shotcode.php

<?php
/**
 * Plugin Name: Snippets Shortcode
 * Plugin URI: http://wordpress.stackexchange.com/q/116044/12615
 * Description: Add code snippets as a Custom Post Type. Display in regular posts and pages using a Shortcode. Rendered with Google Prettify in frontend. 
 * Version: 1.0
 * Author: Rodolfo Buaiz
 * Author URI: http://brasofilo.com
 * License: GPLv2 or later
 *  
 *
 * 
 * This program is free software; you can redistribute it and/or modify it 
 * under the terms of the GNU General Public License version 2, 
 * as published by the Free Software Foundation.  You may NOT assume 
 * that you can use any other version of the GPL.
 *
 * This program is distributed in the hope that it will be useful, 
 * but WITHOUT ANY WARRANTY; without even the implied warranty 
 * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 */

/**
 * Based on Plugin Class Demo
 * https://gist.github.com/toscho/3804204
 */
add_action(
    'plugins_loaded', 
    array( B5F_Snippets_Shortcode::get_instance(), 'plugin_setup' )
);

/**
 * Main class
 *
 * Fires all classes
 * Handles save_post action for other classes
 *
 */
class B5F_Snippets_Shortcode
{
    protected static $instance = NULL;
    public $post_type = 'snippet';

    public function plugin_setup()
    {
        new B5F_SS_Cpt();
        new B5F_SS_Posts_Pages_Metabox();
        new B5F_SS_Shortcode();
        add_action( 'save_post', array( $this, '_save_post' ), 10, 2 );
    }


    public function __construct() {}

    public function get_instance()
    {
        NULL === self::$instance and self::$instance = new self;
        return self::$instance;
    }

    public function _save_post( $post_id, $post_object )
    {
        if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
            return;

        if(
            !isset( $_POST['_nonce_snippets'] ) 
            || !wp_verify_nonce( $_POST['_nonce_snippets'], plugin_basename( __FILE__ ) )
        )
            return;

        switch( $post_object->post_type )
        {
            case $this->post_type:
                if( isset( $_POST['snipp'] ) )
                    update_post_meta( $post_id, '_snippet_code', $_POST['snipp'] );
                break;
            case 'post':
            case 'page':
                if( isset( $_POST['_snippet_shortcode'] ) )
                    update_post_meta( $post_id, '_snippet_shortcode', $_POST['_snippet_shortcode'] );
                else
                    delete_post_meta( $post_id, '_snippet_shortcode' );
                break;
        }
    }
}

/**
 * Register Snippet post type and render Textarea Field after the Title
 *
 * CPT is hierarchical 
 * CPT menu shows up inside Post post type (cool!)
 * Custom Metabox for CPT is left empty, can be used as Language Selector
 *
 */
class B5F_SS_Cpt
{
    public function __construct()
    {
        add_action( 'init', array( $this, '_cpt' ) );
        add_action( 'edit_form_after_title', array( $this, 'input_text_area' ) );
    }

    public function _cpt()
    {
        $labels = array(
            'menu_name'          => 'Snippets',
            'singular_name'      => 'Snippet',
            'add_new'            => 'Add snippet',
            'add_new_item'       => 'Add new snippet',
            'edit_item'          => 'Edit snippet',
            'new_item'           => 'New snippet',
            'view_item'          => 'View snippet',
            'search_items'       => 'Search snippet',
            'not_found'          => 'Snippet not fount',
            'not_found_in_trash' => 'No snippet in trash',
            'parent_item_colon'  => 'Parent snippet',
        );
        $args = array(
            'labels'               => $labels,
            'public'               => false,
            'publicly_queryable'   => false,
            'show_ui'              => true,
            'show_in_menu'         => 'edit.php',
            'query_var'            => true,
            'rewrite'              => false,
            'capability_type'      => 'post',
            'has_archive'          => false,
            'hierarchical'         => true,
            'menu_position'        => null,
            'supports'             => array( 'title', 'page-attributes' ),
            'register_meta_box_cb' => array( $this, '_meta_box' )
        );
        register_post_type( B5F_Snippets_Shortcode::get_instance()->post_type, $args );
    }

    public function _meta_box() { /* add_meta_box(); */ }

    public function input_text_area( $post )
    {
        if( B5F_Snippets_Shortcode::get_instance()->post_type != $post->post_type )
            return;

        $option = ( $get = get_post_meta( $post->ID, '_snippet_code', true ) ) ? $get : '';
        wp_nonce_field( plugin_basename( __FILE__ ), '_nonce_snippets' );
        printf(
            '<h3>%s</h3><textarea id="snipp" name="snipp" cols="90" rows="25" class="widefat">%s</textarea>', 
            __( 'Code' ), 
            esc_html( $option )
        );
    }
}

/**
 * Metabox to select the Snippet and to display the correspondent Shortcode
 *
 * Hardcoded post types in _meta_boxes()
 * Hardcoded jQuery to listen to the Snippets Dropdown changes and update the sample Text Field
 *
 */
class B5F_SS_Posts_Pages_Metabox
{
    public function __construct()
    {
        add_action( 'add_meta_boxes', array( $this, '_meta_boxes' ) );
    }

    public function _meta_boxes()
    {
        foreach( array( 'post', 'page' ) as $pt )
            add_meta_box(
                'snippets_mb', 
                __( 'Snippets' ), 
                array( $this, '_snippets_mb' ), 
                $pt, 
                'side'
            );
    }

    function _snippets_mb( $post, $box )
    {
        $args = array(
            'numberposts' => -1,
            'post_type'   => B5F_Snippets_Shortcode::get_instance()->post_type,
            'post_status' => 'publish'
        );
        $get_posts = get_posts( $args );
        $saved = get_post_meta( $post->ID, '_snippet_shortcode', true );
        wp_nonce_field( plugin_basename( __FILE__ ), '_nonce_snippets' );
        echo '<style>#_snippet_shortcode{width:100%}</style>'; # LAME FIX !!!!!
        wp_dropdown_pages( array(
            'post_type'        => 'snippet',
            'selected'         => $saved,
            'name'             => '_snippet_shortcode',
            'id'               => '_snippet_shortcode',
            'show_option_none' => '- Select -'
        ));
        echo '<hr /><input type="text" class="widefat" id="render-shortcode" value="" />';
        ?>
            <script type="text/javascript">
            jQuery(document).ready(function( $ )
            {
                function do_the_shortcode( val )
                {
                    var full_shortcode = ( val != '' ) ? '[snippet id="' + val + '"]' : '';
                    $('#render-shortcode').val( full_shortcode );
                }

                $('#_snippet_shortcode').change( function()
                { 
                    do_the_shortcode( $(this).val() );
                });

                do_the_shortcode( $('#_snippet_shortcode').val() );
            });
            </script>
        <?php
    }
}

/**
 * Snippets Shortcode
 *
 * First gets the Snippet Post ID, then the associated Code Meta Data, 
 * finally filters the string with http://php.net/manual/en/function.htmlentities.php 
 *
 * Uses Google Code Prettify
 * https://code.google.com/p/google-code-prettify/
 */
class B5F_SS_Shortcode
{
    public function __construct()
    {
        add_shortcode( 'snippet', array( $this, '_shortcode' ) );
    }

    public function _shortcode()
    {
        global $post;

        $file = 'run_prettify.js?lang=css&skin=sons-of-obsidian';
        wp_enqueue_script(
            'prettify-js', 
            'https://google-code-prettify.googlecode.com/svn/loader/' . $file, 
            array()
        );

        $snippet_id = get_post_meta( $post->ID, '_snippet_shortcode', true );
        $code = get_post_meta( $snippet_id, '_snippet_code', true );
        $snippet_info = get_post( $snippet_id );
        return sprintf(
            '<h3>%s</h3><pre class="prettyprint linenums">%s</pre>', 
            $snippet_info->post_title, 
            htmlentities( $code, ENT_QUOTES )
        );
    }
}