mbstring.func_overload can only be set in php.ini, so set value to 2 or greater.

Having a valid multibyte key (since ZBX-4218) for example ' log[/var/log/syslog,\'???\',"UTF-8",100] ', adding it to trigger will produce error like ' Incorrect item key "log[/var/log/syslog,\'???\',"UTF-8",100].last(" '.

For 2.2 you can also create a web scenario with name like "åäö" or "???". Then create a trigger and select one of the created web items. For example ' Download speed for scenario "åäö" '.
Using frontend pop-up will display error: ' Incorrect trigger function "t()}=0" provided in expression. Unknown function. ' or you can enter expression manually and displayed error will be ' Incorrect item key "web.test.in[???,,bps].last(" '.

Having mbstring.func_overload > 2, instead of substr() mb_substr() function will be used. Due to mb_ function cutting expression somewhere else, it cannot correctly determine expression parts which one is the key and which one is the function etc.