我正在使用Ubuntu 17.04和PHP 7.0，当我尝试安装xml模块时，我遇到了问题。
如果我尝试使用此命令安装xml模块......
sudo apt-get install php7.0-xml
我发现了这个错误：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
php7.0-xml : Depends: php7.0-common (= 7.0.18-0ubuntu0.17.04.1)
but 7.0.18-1+deb.sury.org~yakkety+1 is to be installed
E: Unable to correct problems, you have held broken packages.
如果我尝试安装php7.0-common，我收到了这条消息：
php7.0-common is already in its most recent version
(7.0.18-1+deb.sury.org~yakkety+1).
那么，我该如何安装xml模块？