Actually with further digging I found that Phifer didn't take command of Roane's brigade until 25 May:
"On April 24, Roane's brigade was actually commanded by Lieutenant Colonel Benjamin F. Danley (3rd Arkansas Cavalry Regiment, Dismounted), and on May 25 Van Dorn assigned acting brigadier general Charles W. Phifer to head the brigade." from pg. 254 of Lawrence Hewitt's and Arthur Bergeron's Confederate Generals in the Western Theater

So it looks like Lieut. Col. Benjamin Danley was in command of Roane's brigade first, then in late May Van Dorn assigned Phifer to command it.