/** 
 * 当前是否ssl 
 * @return bool 
 */
function is_ssl(){    
    if (isset($_SERVER['HTTPS']) && ('1' == $_SERVER['HTTPS'] || 'on' == strtolower($_SERVER['HTTPS']))) {        
        return true;    
    } elseif (isset($_SERVER['HTTP_FRONT_END_HTTPS']) && 'on' == strtolower($_SERVER['HTTP_FRONT_END_HTTPS'])) {        return true;    
    } elseif (isset($_SERVER['HTTP_X_CLIENT_SCHEME']) && 'https' == $_SERVER['HTTP_X_CLIENT_SCHEME']){        
        return true;    
    } elseif (isset($_SERVER['REQUEST_SCHEME']) && 'https' == $_SERVER['REQUEST_SCHEME']) {        
        return true;    
    } elseif (isset($_SERVER['SERVER_PORT']) && '443' == $_SERVER['SERVER_PORT']) {        
        return true;    
    } elseif (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && 'https' == $_SERVER['HTTP_X_FORWARDED_PROTO']) {        
        return true;    
    }    
    return false;
}