We see hundreds of phpadmin and similar requests each day, all trying to hack in. Unless you actually have phpadmin on your server it pays to block all accesses to that path. In my case I do not use php at all, so I can also block any script ending in .php.
As to the IP range - I have a LOT of softlayer ranges blocked - in fact, any IP range that looks anything like a server farm.
If the issue is new to you then learn how to block user-agents, scripts and IPs, then look through this forum for IP ranges and user-agents to block - there are hundreds of them! :)
And then there are the other headers...