$ sudo apt-get update
$ sudo apt-get install php-htmlpurifier 

php-htmlpurifier is installed in your system.

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

$ sudo dpkg-query -l | grep php-htmlpurifier *

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

php-htmlpurifier package basic information: