我在 Ubuntu Server 12.04.5 上我正在尝试安装 php5-mcrypt。我收到这个错误。
~$ sudo apt-get install php5-mcrypt
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package php5-mcrypt is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'php5-mcrypt' has no installation candidate
我该如何解决这个问题？