WordPressに独自のページを表示する

class-addpage.php

<?php

if (!class_exists('WP_AddPage')):
class WP_AddPage {

private $rule      = null;
private $title     = null;
private $callback  = null;
private $template  = null;
private $filter    = false;

function __construct($rule, $title, $callback)
{
    if (!class_exists('WP_AddRewriteRules')) {
        wp_die("Class WP_AddRewriteRules is not exists.");
    }
    $this->rule  = $rule;
    $this->title = $title;
    $this->callback  = $callback;
}

public function init()
{
    new WP_AddRewriteRules(
        $this->rule,
        'custom-'.preg_replace('/\W/', '', $this->rule),
        array(&$this, "_display")
    );
}

public function set_filter($filter = false)
{
    $this->filter = $filter;
    return true;
}

public function set_template($template)
{
    $this->template = $template;
    return true;
}


public function _page_template($template)
{
    return $this->template;
}

public function _display()
{
    if ($this->template) {
        add_filter('page_template', array(&$this, "_page_template"));
    }

    $content = call_user_func($this->callback);

    if ($this->filter === false) {
        global $wp_filter;
        unset($wp_filter['the_content']);
    }

    global $wp_query;
    $wp_query->is_404 = null;
    $wp_query->is_page = null;
    $wp_query->is_single = null;
    $wp_query->is_singular = 1;
    $wp_query->is_search = null;
    $wp_query->is_archive = null;
    $wp_query->is_date = null;
    $wp_query->is_day = null;
    $wp_query->is_home = null;
    $wp_query->post_count = 1;
    $wp_query->post->post_title = $this->title;
    $wp_query->posts = array();
    $wp_query->posts[0] = new stdClass();
    $wp_query->posts[0]->ID = 0;
    $wp_query->posts[0]->post_author = 1;
    $wp_query->posts[0]->post_title = $this->title;
    $wp_query->posts[0]->comment_status = 'close';
    $wp_query->posts[0]->comment_count = 0;
    $wp_query->posts[0]->ping_status = 'close';
    $wp_query->posts[0]->post_type = 'custom-page';
    $wp_query->posts[0]->post_status = 'publish';
    $wp_query->posts[0]->post_parent = 0;
    $wp_query->posts[0]->post_content = $content;
    $wp_query->posts[0]->post_date = time();
}

}
endif;

// EOF

example.php

<?php

// 例 wpより前のフックで以下のようなコードを実行。

$cpanel = new WP_AddPage(
    'cpanel$',
    'コントロールパネル',
    array(&$this, "cpanel")
);

// wp_contentフィルターを適用する場合はtrueをセットする。
$cpanel->set_filter(true);

// テーマファイル内にpage-cpanel.phpがあればそれを使用して表示する。
// デフォルトでは固定ページ用のテンプレート(page.php)
$cpanel->set_template("page-cpanel.php");

$cpanel->init(); // カスタムページを登録

function cpanel() {

    // apiにアクセスしたりデータベースから値をとったり何でもござれ！

    return "カスタムコンテンツ";

}

以下のクラスも必要。
https://gist.github.com/1254007