Barnsley looked to have had a strong early shout for a penalty when Reece James collided with Phillips but the appeals were turned down.
The home side finished the first half strongly with Mallik Wilks, a one-time Red, having a dangerous shot deflected into Jordan Hugill's path but the striker was unable to bundle it on target.
Barnsley did get a penalty after 52 minutes when Davis Keillor-Dunn latched onto James' header and was then felled by goalkeeper Dillon Phillips, who was sent the wrong way by Phillips.
Rotherham's Zak Jules then looked very fortunate to get away with a handball in the box while Phillips tried to wrap up the points with a long-range effort which had to be tipped wide by Phillips.