cannot save wp-config.php

BPS would not prevent you from editing your wp-config.php file and saving those changes. Check that you have set the file permissions to 644 so that you can edit and save your changes to the wp-config.php file. If you are unable to change the file permissions or if you change the file permissions to 644 and your changes are not saved then your Host might be restricting your from editing your wp-config.php file.

Or since you posted over here – http://ww.wp.xz.cn/support/topic/text-javascript-unknown-inserted-into-my-headerphp-1?replies=3 – that your website is already hacked then it is possible that the hacker is restricting you from editing the wp-config.php file.