I think that it's obvious that =net-analyzer/zabbix-2.2.5 with USE="frontend mysql" should RDEPEND on dev-lang/php with USE=mysqli .
Just stumbled over this one after searching for hours why my zabbix install would not display the web gui. I had the mysql USE flag enabled in php but not the msyqli USE flag. This is really my fault but a proper USE declaration in the zabbix ebuild would have saved me a lot of time.
zabbix-4.4.0.ebuild looks to have proper requirements