If the 1860 census lists him as a resident of Kosciusko, then it is probable he was a member of the 13th Mississippi, since the 13th's Co.I/D were raised in Attala County, the county seat of which is Kosciusko.

No elements of the 18th Mississippi were raised in Attala County. Also, Grady Howell's Volume I, page 374 shows,

Burt, Phillip M.; Pvt., Sgt.; I; 13th Regt Inf.