Well, on openSUSE 12.3 the needed package is called “php5-sockets” and is included in the standard distribution.
Don’t know about SLES though.
Are you using the standard php that is included?
Then just start YaST->Software Management and type “php5-sockets” into the search field.
A package should be found, install that.
You may have to restart apache afterwards (or reboot), I’m not sure right now.
While it is possible you get the ultimate answer here, it more likely you get an answer here from people that do not even know that someting like SLES/SLED exists. Thus such an answer could not be applicable/trustable for your case.
I just had a look at the cacti package on OBS, and it does require “php5-sockets”, so that should automatically get installed when you install the cacti package.
So maybe you are using a self-compiled PHP, so the system’s “php5-sockets” package isn’t picked up?