哪位高手能说明一下discuz X2.5 upload/index.php文件思路！

<?php



/**

 *      [Discuz!] (C)2001-2099 Comsenz Inc.

 *      This is NOT a freeware, use is subject to license terms

 *

 *      $Id: index.php 27617 2012-02-07 08:24:14Z monkey $

 */



if(!empty($_SERVER['QUERY_STRING']) && is_numeric($_SERVER['QUERY_STRING'])) {

	$_ENV['curapp'] = 'home';

	$_GET = array('mod'=>'space', 'uid'=>$_SERVER['QUERY_STRING']);

} else {



	$url = '';

	$domain = $_ENV = array();

	$jump = false;

	@include_once './data/sysdata/cache_domain.php';

	$_ENV['domain'] = $domain;

	if(empty($_ENV['domain'])) {

		$_ENV['curapp'] = 'forum';

	} else {

		$_ENV['defaultapp'] = array('portal.php' => 'portal', 'forum.php' => 'forum', 'group.php' => 'group', 'home.php' => 'home');

		$_ENV['hostarr'] = explode('.', $_SERVER['HTTP_HOST']);

		$_ENV['domainroot'] = substr($_SERVER['HTTP_HOST'], strpos($_SERVER['HTTP_HOST'], '.')+1);

		if(!empty($_ENV['domain']['app']) && is_array($_ENV['domain']['app']) && in_array($_SERVER['HTTP_HOST'], $_ENV['domain']['app'])) {

			$_ENV['curapp'] = array_search($_SERVER['HTTP_HOST'], $_ENV['domain']['app']);

			if($_ENV['curapp'] == 'mobile') {

				$_ENV['curapp'] = 'forum';

				if(@$_GET['mobile'] != 'no') {

					@$_GET['mobile'] = 'yes';

				}

			}

			if($_ENV['curapp'] == 'default' || !isset($_ENV['defaultapp'][$_ENV['curapp'].'.php'])) {

				$_ENV['curapp'] = '';

			}

		} elseif(!empty($_ENV['domain']['root']) && is_array($_ENV['domain']['root']) && in_array($_ENV['domainroot'], $_ENV['domain']['root'])) {



			$_G['setting']['holddomain'] = $_ENV['domain']['holddomain'] ? $_ENV['domain']['holddomain'] : array('www');

			$list = $_ENV['domain']['list'];

			if(isset($list[$_SERVER['HTTP_HOST']])) {

				$domain = $list[$_SERVER['HTTP_HOST']];

				$id = intval($domain['id']);

				switch($domain['idtype']) {

					case 'subarea':

						$_ENV['curapp'] = 'forum';

						$_GET['gid'] = $id;

						break;

					case 'forum':

						$_ENV['curapp'] = 'forum';

						$_GET['mod'] = 'forumdisplay';

						$_GET['fid'] = $id;

						break;

					case 'topic':

						$_ENV['curapp'] = 'portal';

						$_GET['mod'] = 'topic';

						$_GET['topicid'] = $id;

						break;

					case 'channel':

						$_ENV['curapp'] = 'portal';

						$_GET['mod'] = 'list';

						$_GET['catid'] = $id;

						break;

				}

			} elseif(count($_ENV['hostarr']) > 2 && $_ENV['hostarr'][0] != 'www' && !checkholddomain($_ENV['hostarr'][0])) {

				$_ENV['prefixdomain'] = addslashes($_ENV['hostarr'][0]);

				$_ENV['domainroot'] = addslashes($_ENV['domainroot']);

				require_once './source/class/class_core.php';

				C::app()->init_setting = true;

				C::app()->init_user = false;

				C::app()->init_session = false;

				C::app()->init_cron = false;

				C::app()->init_misc = false;

				C::app()->init();

				$jump = true;

				$domain = C::t('common_domain')->fetch_by_domain_domainroot($_ENV['prefixdomain'], $_ENV['domainroot']);

				$apphost = $_ENV['domain']['app'][$domain['idtype']] ? $_ENV['domain']['app'][$domain['idtype']] : $_ENV['domain']['app']['default'];

				$apphost = $apphost ? 'http://'.$apphost.'/' : '';

				switch($domain['idtype']) {

					case 'home':

						if($_G['setting']['rewritestatus'] && in_array('home_space', $_G['setting']['rewritestatus'])) {

							$url = rewriteoutput('home_space', 1, $apphost, $domain['id']);

						} else {

							$url = $apphost.'home.php?mod=space&uid='.$domain['id'];

						}

						break;

					case 'group':

						if($_G['setting']['rewritestatus'] && in_array('group_group', $_G['setting']['rewritestatus'])) {

							$url = rewriteoutput('group_group', 1, $apphost, $domain['id']);

						} else {

							$url = $apphost.'forum.php?mod=group&fid='.$domain['id'].'&page=1';

						}

						break;

				}

			}

		} else {

			$jump = true;

		}

		if(empty($url) && empty($_ENV['curapp'])) {

			if(!empty($_ENV['domain']['defaultindex']) && !$jump) {

				if($_ENV['defaultapp'][$_ENV['domain']['defaultindex']]) {

					$_ENV['curapp'] = $_ENV['defaultapp'][$_ENV['domain']['defaultindex']];

				} else {

					$url = $_ENV['domain']['defaultindex'];

				}

			} else {

				if($jump) {

					$url = empty($_ENV['domain']['app']['default']) ? (!empty($_ENV['domain']['defaultindex']) ? $_ENV['domain']['defaultindex'] : 'forum.php') : 'http://'.$_ENV['domain']['app']['default'];

				} else {

					$_ENV['curapp'] = 'forum';

				}

			}

		}

	}

}

if(!empty($url)) {

	$delimiter = strrpos($url, '?') ? '&' : '?';

	if(isset($_GET['fromuid']) && $_GET['fromuid']) {

		$url .= $delimiter.'fromuid='.$_GET['fromuid'];

	} elseif(isset($_GET['fromuser']) && $_GET['fromuser']) {

		$url .= $delimiter.'fromuser='.$_GET['fromuser'];

	}

	header("HTTP/1.1 301 Moved Permanently");

	header("location: $url");

} else {

	require './'.$_ENV['curapp'].'.php';

}



function checkholddomain($domain) {

	global $_G;



	$domain = strtolower($domain);

	if(preg_match("/^[^a-z]/i", $domain)) return true;

	$holdmainarr = empty($_G['setting']['holddomain']) ? array('www') : explode('|', $_G['setting']['holddomain']);

	$ishold = false;

	foreach ($holdmainarr as $value) {

		if(strpos($value, '*') === false) {

			if(strtolower($value) == $domain) {

				$ishold = true;

				break;

			}

		} else {

			$value = str_replace('*', '.*?', $value);

			if(@preg_match("/$value/i", $domain)) {

				$ishold = true;

				break;

			}

		}

	}

	return $ishold;

}

?>