Steps to reproduce:

1. Install zabbix frontend on host-a
2. Install zabbix server  on host-b 
3. Edit zabbix.conf.php and set $ZBX_SERVER='host-b'
4. Leave $ZBX_SERVER_PORT commented (as zabbix server is listening at default port 10051)

Result:

System Information reports that Zabbix server on "localhost:10051" is down.  This is weird because "localhost" is never mentioned in zabbix.conf.php. This message could be misleading and difficult to troubleshoot for users.

The error can be fixed only by explicitly defining a value for $ZBX_SERVER_PORT variable  too. i.e:   $ZBX_SERVER_PORT='10051'

Expected:
Zabbix frontend should honour $ZBX_SERVER, also when $ZBX_SERVER_PORT is not defined. It should not fall back to localhost default.

Alternatively, documentation should clearly state that the two variables needs to be BOTH defined.