$ sudo apt-get update
$ sudo apt-get install php-net-imap 

php-net-imap is installed in your system.

Make ensure the php-net-imap package were installed using the commands given below,

$ sudo dpkg-query -l | grep php-net-imap *

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

php-net-imap package basic information: