Elgg 3 requires PHP 7 (I thinkj Elgg 3.1 even PHP 7.1) at minimum. I never tried out HHVM so I can't tell if Elgg ever worked with HHVM at all. But as far as I know HHVM is no longer supposed to be 100% compatible with PHP (or at least in future it might no longer be). See https://hhvm.com/blog/2018/09/12/end-of-php-support-future-of-hack.html. Maybe you can get it working (also compatibility with PHP 7) setting the flag as described in https://hhvm.com/blog/2018/09/12/end-of-php-support-future-of-hack.html. But I doubt that it makes any sense to try getting Elgg to work on HHVM for a production site because even if it works now it might not in the near future with PHP (and Elgg) and HHVM drifting in different directions.

Yes, Elgg 1.11 requires the PHP extension ext/mysql. Elgg 2.x uses PDO MySQL.

What type of illegal content? Do you mean offensive words? There should be "Bad Word" something plugin out there. I think Elgg has one in its repository in one of the spam tools.

you can use report content. for that user can report you, 

Diphen,

Do you mean a filter for copywrited material, similar to YouTube';s Content ID ? 

https://support.google.com/youtube/answer/2797370?hl=en