An example of using Firebase and WordPress together. The idea is to use WP's custom post types and metaboxes to make content management easy, and sync with Firebase so that your websites have access to a real-time JSON feed of your custom data.

wordpress-firebase.php

<?php

/**
 * All custom functions should be defined in this class
 * and tied to WP hooks/filters w/in the constructor method
 */
class Custom_Functions {

  // Custom metaboxes and fields configuration

  public $metabox_namespace = 'cmb';
  public $metabox_prefix    = '_cmb_';


  // Firebase configuration

  public $firebase_url = 'https://burning-fire-9735.firebaseio.com';
  public $firebase_key = 'KEYGOESHERE';


  // Need to list wysiwyg metaboxes here so we can apply the
  // appropriate filters

  public $wysiwyg_fields = array(
    'title',
    'contact',
    'bio'
  );


  public function __construct() {

    // Include a helper library for adding custom metaboxes
    // 
    // https://github.com/WebDevStudios/Custom-Metaboxes-and-Fields-for-WordPress
    
    add_action('after_setup_theme', array(
      $this, 'include_custom_metabox_helper'
    ));


    // Add the "Speaker" and "Presentation" post types
    
    add_action('registered_post_type', array(
      $this, 'add_custom_post_types'
    ));


    // Define custom metaboxes to add to "edit Speaker" 
    // and "edit Presentation" pages in WP
    
    add_filter('cmb_meta_boxes', array(
      $this, 'add_metaboxes'
    ));


    // Send "Speaker" and "Presentation" metadata to Firebase
    // 
    // https://github.com/ktamas77/firebase-php

    add_action('save_post', array(
      $this, 'send_updated_metadata_to_firebase'
    ));


    // Remove unnecessary admin menus
    
    add_action('admin_menu', array(
      $this, 'remove_unneccessary_admin_menus'
    ));

  }


  // Sends speaker data (including presentations) to Firebase
  // whenever the speaker or one of his presentations is 
  // updated in WP

  public function send_updated_metadata_to_firebase($post_id) {
    if (wp_is_post_revision($post_id)) return;

    $speaker_id = $this::get_speaker_id_for_post($post_id);
   
    $data = $this::get_speaker_object($speaker_id, $this->metabox_prefix);

    $this::send_to_firebase($speaker_id, $data);
  }


  // An associative array of all of the speakers, including their
  // personal info and presentation info

  public function get_speaker_object($speaker_id) {
    $speakers_post_meta = get_post_meta($speaker_id);

    $data = $this::get_processed_metadata($speaker_id);

    $data['name']          = get_the_title($speaker_id);
    $data['presentations'] = $this::get_presentation_metadata($speaker_id);

    return $data;
  }


  public function include_custom_metabox_helper() {
    $this::include_helper('metaboxes-and-fields/init.php');
  }


  public function add_custom_post_types() {
    $this::include_helper('post-type-helper.php');

    $args = array('supports' => array('title'));

    $speakers      = new Custom_Post_Type('speaker', $args);
    $presentations = new Custom_Post_Type('presentation', $args);
  }


  public function add_metaboxes(array $metaboxes) {
    $this::include_helper('metaboxes-and-fields/init.php');

    // Add metaboxes to the "Speaker" custom post type

    $metaboxes['speaker_metaboxes'] = array(

      'id'         => 'speaker_metaboxes',
      'title'      => __('Speaker Info', $this->metabox_namespace),
      'pages'      => array('speaker'),
      'context'    => 'normal',
      'priority'   => 'core',
      'show_names' => true,
      'fields'     => array(

        // speaker's headshot photo

        array(
          'name' => __('Headshot Photo', $this->metabox_namespace),
          'id'   => $this->metabox_prefix . 'speaker_headshot',
          'type' => 'file'
        ),

        // speaker's job title

        array(
          'name' => __('Job Title', $this->metabox_namespace),
          'id'   => $this->metabox_prefix . 'speaker_title',
          'type' => 'wysiwyg',
          'options' => array('textarea_rows' => 2)
        ),

        // speaker's contact info

        array(
          'name' => __('Contact Info', $this->metabox_namespace),
          'id'   => $this->metabox_prefix . 'speaker_contact',
          'type' => 'wysiwyg',
          'options' => array('textarea_rows' => 5)
        ),

        // speaker's bio

        array(
          'name' => __('Bio', $this->metabox_namespace),
          'id'   => $this->metabox_prefix . 'speaker_bio',
          'type' => 'wysiwyg',
          'options' => array('textarea_rows' => 14)
        )

      )

    );

    // Now we'll add metaboxes to the "Presentation" post type
    // 
    // But first we need an array of speakers so we can assign
    // a speaker to each presentation...

    $speakers_dropdown_array = $this::get_speakers_dropdown_array();

    // Let's get on with it!

    $metaboxes['presentation_metaboxes'] = array(

      'id'         => 'presentation_metaboxes',
      'title'      => __('Presentation Info', $this->metabox_namespace),
      'pages'      => array('presentation'),
      'context'    => 'normal',
      'priority'   => 'core',
      'show_names' => true,
      'fields'     => array(

        // the speaker

        array(
          'name'    => __('Speaker', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_speaker',
          'type'    => 'select',
          'options' => $speakers_dropdown_array
        ),

        // the presentation date

        array(
          'name'    => __('Date', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_date',
          'type'    => 'text_date'
        ),

        // start time

        array(
          'name'    => __('Start Time', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_start_time',
          'type'    => 'text_time'
        ),

        // end time

        array(
          'name'    => __('End Time', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_end_time',
          'type'    => 'text_time'
        ),

        // video ID (unique part of the embed code)

        array(
          'name'    => __('Video ID', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_video_id',
          'type'    => 'text_medium'
        ),

        // video host (wistia or livestream.com?)

        array(
          'name'    => __('Video Host', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_video_host',
          'type'    => 'select',
          'options' => array(
            'wistia'     => __('Wistia',         $this->metabox_namespace),
            'livestream' => __('Livestream.com', $this->metabox_namespace)
          )
        ),

        // banner ad

        array(
          'name' => __('Banner', $this->metabox_namespace),
          'id'   => $this->metabox_prefix . 'presentation_banner',
          'type' => 'file'
        ),

        // promo URL for banner ad

        array(
          'name' => __('Banner URL', $this->metabox_namespace),
          'id'   => $this->metabox_prefix . 'presentation_banner_url',
          'type' => 'text_url'
        ),

        // collection of live blog entries

        array(
          'name'    => __('Live Blog Entries', $this->metabox_namespace),
          'id'      => $this->metabox_prefix . 'presentation_liveblog',
          'type'    => 'group',
          'options' => array(
            'add_button'    => __('Add Another Entry', 'cmb'),
            'remove_button' => __('Remove Entry', 'cmb'),
            'sortable'      => false
          ),
          'fields' => array(
            array(
              'name' => __('Live Blog Entry', $this->metabox_namespace),
              'id'   => $this->metabox_prefix . 'presentation_liveblog_entry',
              'type' => 'wysiwyg',
              'options' => array('textarea_rows' => 4)
            )
          )
        )

      )

    );

    return $metaboxes;
  }


  public function remove_unneccessary_admin_menus() {
    remove_menu_page('index.php');
    remove_menu_page('link-manager.php');
    remove_menu_page('edit-comments.php');
    remove_menu_page('customize.php');
    remove_menu_page('theme-editor.php');
    remove_menu_page('users.php');
    remove_menu_page('tools.php');
    remove_menu_page('plugins.php');
    remove_menu_page('edit.php');
    remove_menu_page('edit.php?post_type=page');
    remove_menu_page('upload.php');
  }


  private function get_presentation_metadata($speaker_id) {
    $presentations = array();

    $presentation_query = new WP_Query(array(
      'post_type'  => 'presentation',
      'meta_value' => $speaker_id
    ));

    if ($presentation_query->have_posts()) {
      while ($presentation_query->have_posts()) {
        $presentation_query->the_post();

        $presentation_title = get_the_title();

        $presentations[$presentation_title] = $this::get_processed_metadata(get_the_ID());
      }
    }

    return $presentations;
  }


  private function get_processed_metadata($post_id) {
    $metadata = get_post_meta($post_id);

    $metadata = $this::remove_unneccessary_metadata($metadata);
    $metadata = $this::add_video_embed_to_metadata($metadata);
    $metadata = $this::add_liveblogs_to_metadata($metadata, $post_id);
    $metadata = $this::strip_prefixes_from_metadata($metadata);
    $metadata = $this::reformat_metadata_array($metadata);
    $metadata = $this::format_wysiwyg_metadata($metadata);

    return $metadata;
  }


  private function add_liveblogs_to_metadata($metadata, $post_id) {
    if ($this::is_speaker($post_id))
      return $metadata;

    $liveblog_entries  = array();
    $liveblog_metabox  = $this->metabox_prefix . 'presentation_liveblog';
    $liveblog_postmeta = get_post_meta($post_id, $this->metabox_prefix . 'presentation_liveblog', false);

    foreach ($liveblog_postmeta as $group=>$entries) {
      foreach ($entries as $key=>$entry) {
        $liveblog_entries[] = wpautop($entry[$this->metabox_prefix . 'presentation_liveblog_entry']);
      }
    }

    $metadata[$liveblog_metabox] = $liveblog_entries;

    return $metadata;
  }


  private function remove_unneccessary_metadata($metadata) {
    foreach ($metadata as $fieldname=>$value) {
      if ($this::is_an_unnecessary_metabox_field($fieldname)) {
        unset($metadata[$fieldname]);
      }
    }

    return $metadata;
  }


  public function is_an_unnecessary_metabox_field($fieldname) {
    if (!$this::is_a_custom_metabox($fieldname)) return true;
    if (strstr($fieldname, 'headshot_id'))       return true;

    return false;
  }


  private function add_video_embed_to_metadata($metadata) {
    if ($this::has_video($metadata)) {
      $metadata['video'] = $this::get_video_embed($metadata);

      unset($metadata['video_id']);
      unset($metadata['video_host']);
    }

    return $metadata;
  }


  private function strip_prefixes_from_metadata($metadata) {
    foreach ($metadata as $old_fieldname=>$value) {
      $new_fieldname = $this::strip_prefix_from_fieldname($old_fieldname);

      $metadata[$new_fieldname] = $metadata[$old_fieldname];

      unset($metadata[$old_fieldname]);
    }

    return $metadata;
  }


  private function reformat_metadata_array($metadata) {
    foreach ($metadata as $fieldname=>$value) {
      if ($fieldname === 'liveblog') continue;

      $metadata[$fieldname] = $metadata[$fieldname][0];
    }

    return $metadata;
  }


  private function format_wysiwyg_metadata($metadata) {
    foreach ($metadata as $fieldname=>$value) {
      if (in_array($fieldname, $this->wysiwyg_fields)) {
        $metadata[$fieldname] = wpautop($value);
      }
    }

    return $metadata;
  }


  private function has_video($metadata) {
    return !(empty($metadata['video_host']) || empty('video_id'));
  }


  private function get_video_embed($metadata) {
    if ($metadata['video_host'] === 'wistia') {
      return '<iframe src="' . $metadata['video_id'] . '">';
    }

    if ($metadata['video_host'] === 'livestream') {
      return '<div id="' . $metadata['video_id'] . '">';
    }

    return null;
  }


  private function is_a_custom_metabox($fieldname) {
    return (strstr($fieldname, $this->metabox_prefix));
  }


  private function strip_prefix_from_fieldname($fieldname) {
    $fieldname = str_replace($this->metabox_prefix, '', $fieldname);

    $post_type_prefix = '/^speaker_|^presentation_/';
    
    return preg_replace($post_type_prefix, '', $fieldname);
  }


  private function get_speakers_dropdown_array() {
    $dropdown_array = array();

    $query = new WP_Query(array('post_type' => 'speaker'));
    
    if ($query->have_posts()) {
      while ($query->have_posts()) {
        $query->the_post();

        $title = get_the_title();
        $key   = get_the_ID();

        $dropdown_array[$key] = __($title, $this->metabox_namespace);
      }
    }

    return $dropdown_array;
  }


  private function send_to_firebase($key, $value) {
    $this::include_helper('firebase-php/firebaseLib.php');

    $firebase = new Firebase($this->firebase_url, $this->firebase_key);

    $firebase->set($key, $value);
  }


  private function get_speaker_id_for_post($post_id) {
    if ($this::is_speaker($post_id)) return $post_id;

    if ($this::is_presentation($post_id)) {
      return intval(get_post_meta($post_id, $this->metabox_prefix . 'presentation_speaker', true));
    }

    return null;
  }


  private function is_speaker($post_id) {
    return (get_post_type($post_id) === 'speaker');
  }


  private function is_presentation($post_id) {
    return (get_post_type($post_id) === 'presentation');
  }

  
  private function include_helper($helper_path) {
    require_once locate_template("/lib/$helper_path");
  }

}

new Custom_Functions; // run all our custom functions