1.12.12 don't work with PHP7

And how I can install PHP5 on Ubuntu 16.04? PHP5 is not aviable on 16.04.

I'm not using Ubuntu, so I can only refer to https://by-example.org/ubuntu-16-04-xenial-downgrade-php-7-to-php-5-6/. Or you might google for instructions on how to install php5 on Ubuntu 16.04.

Or you can try it with Elgg 2.2 on php7. I think there were some fixes added recently to the Elgg 2 branch for better php7 compatibility. I can't say for sure if all issues that might occur with Elgg on php7 are fixed now. Also, there might be compatibility issues with 3rd party plugins to be used on php7. So, it might be better for now to still use php5 (version 5.6).

You might also avoid using mysql version 5.7 for now and stick with mysql version 5.6 as there are also some issue showing up only on mysql version 5.7.

siehe https://elgg.org/discussion/view/2527169/php-mysql-problem#elgg-object-2527348.