执行 apt-get install php-soap 时未满足依赖关系

Question

我正在尝试php-soap在 Ubuntu 14.04 上安装（以 root 身份）。

我执行了apt-get update和apt-get install php-soap，并得到了以下输出：

Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies.
 linux-image-extra-3.13.0-115-generic : Depends: linux-image-3.13.0-115-generic but it is not going to be installed
 linux-image-generic : Depends: linux-image-3.13.0-115-generic but it is not going to be installed
 php-soap : Depends: php7.1-soap but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

我的 PHP 版本是 5.6.xx，所以我不想要依赖于 php7.1 的 soap，正如所提到的。我也不知道这linux-image-3.13.0-115-generic和 soap 有什么关系。有没有办法指定 php-soap 的版本（即 php 5.6），我该如何处理其他问题？