laravel composer update : the requested PHP extension dom is missing from your system
Installing xml-extension (php7.0-xml in my case) solved the problem
sudo apt-get install php-xml
If PHP version is 7.2, execute the following command
sudo apt-get install php7.2-xml
Managed to fix it with a php version 5.6* :
$ sudo apt-get update
$ sudo apt-get install php5.6-xml