A woman who admitted she stabbed her baby to death while under the influence of the illicit drug phencyclidine (PCP) was sentenced to 12 years in prison. Deputy Dist. Atty. Steven Morgan said he agreed to allow Carolyn Neal, 28, to plead guilty to voluntary manslaughter in the Sept. 9, 1985, slaying of her 22-month-old daughter, Elisa, on condition that she be sentenced to the 11-year maximum term for manslaughter--plus an additional year for using a knife. Police said the child had been removed from Neal’s custody when she was found to be suffering from PCP ingestion, and the stabbing took place at the home of Neal’s mother and stepfather, who had temporary custody.