$ sudo apt-get update
$ sudo apt-get install php-symfony-classloader 

php-symfony-classloader is installed in your system.

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

$ sudo dpkg-query -l | grep php-symfony-classloader *

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

php-symfony-classloader package basic information: