$ sudo apt-get update
$ sudo apt-get install php-horde-imp 

php-horde-imp is installed in your system.

Make ensure the php-horde-imp package were installed using the commands given below,

$ sudo dpkg-query -l | grep php-horde-imp *

You will get with php-horde-imp package name, version, architecture and description in a table

php-horde-imp package basic information: