具体原因请看 更换PHP 5.4以上版本后Dedecms后台登录空白的解决方法

解决办法：

打开include/userlogin.class.php

找到

function exitUser() 
    { 
        ClearMyAddon(); 
        @session_unregister($this->keepUserIDTag); 
        @session_unregister($this->keepUserTypeTag); 
        @session_unregister($this->keepUserChannelTag); 
        @session_unregister($this->keepUserNameTag); 
        @session_unregister($this->keepUserPurviewTag); 
        DropCookie('dedeAdmindir'); 
        DropCookie('DedeUserID'); 
        DropCookie('DedeLoginTime'); 
        $_SESSION = array(); 
    } 

替换为：

function exitUser() 
    { 
        ClearMyAddon(); 
        #@session_unregister($this->keepUserIDTag);         
        $_SESSION[$this->keepUserIDTag] = $this->userID; 
        #@session_unregister($this->keepUserTypeTag);         
        $_SESSION[$this->keepUserTypeTag] = $this->userType; 
        #@session_unregister($this->keepUserChannelTag);         
        $_SESSION[$this->keepUserChannelTag] = $this->userChannel; 
        #@session_unregister($this->keepUserNameTag);         
        $_SESSION[$this->keepUserNameTag] = $this->userName; 
        #@session_unregister($this->keepUserPurviewTag);         
        $_SESSION[$this->keepUserPurviewTag] = $this->userPurview; 
        DropCookie('dedeAdmindir'); 
        DropCookie('DedeUserID'); 
        DropCookie('DedeLoginTime'); 
        $_SESSION = array(); 
    }