我正在尝试在 Ubuntu 14.04 上安装 GMP：
$ sudo apt-get install php5-gmp
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:
php5-gmp : Depends: php5-common (= 5.5.9+dfsg-1ubuntu4.5) but 5.5.17+dfsg-2+deb.sury.org~precise+1 is to be installed
E: Unable to correct problems, you have held broken packages.
为什么不让我安装这个？
不知何故，你已经安装了php-common的特殊版本（= 5.5.17+dfsg-2+deb.sury.org~precision+1）尝试
apt-get remove php5-common
然后重新安装。
您的安装似乎被从某个外星存储库安装php5-common-5.5.17+dfsg-2+deb.sury.org~precise+1软件包破坏了。
尝试从以下方面开始：
sudo apt-search php5
删除所有看起来可疑的内容，然后从官方存储库重新安装它。我认为，简单地删除这个外星common并重新安装gmp可能会解决问题。