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

discuz discuz X2.5 upload/index.php代码如下，主说针对运行步骤（架构思路）说明一下，越详细越好，非常感谢！！！

'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;}?>

回复讨论(解决方案)

$_ENV['curapp'] = 'home'; 不知道是什么意思