Police said Monday that David Williams, 23, had been arrested and charged with both slayings.
Daquan Parker, 17, was shot in the head late Saturday in the 2000 block of South Croskey Street in South Philadelphia, police said.
Parker, of the 2500 block of South Sixth Street, was pronounced dead at the scene. No arrests were reported.
Police also identified the man arrested by an off-duty Chester police officer who witnessed a homicide in West Philadelphia early Sunday.
Quinton Frierson, also known as Quinton Ferguson, 27, was charged with the murder of Victor Rodriguez, 20, who was shot in the head in the 300 block of South 52d Street at 2:34 a.m. Rodriguez later died at the Hospital of the University of Pennsylvania.
The motive was an argument, police said.
Police reported no arrests in the separate shooting deaths early Sunday of Keith Anderson, 25, and a woman whose name was still being withheld Monday.
Early Saturday, medics transported an unresponsive 5-year-old boy from his home in the 600 block of North 13th Street to Hahnemann University Hospital, where he was pronounced dead at 1:27 a.m.
Dashawn Harris suffered burns and bruises to his body, police said.
His mother, Lashay Patterson, 23, and her boyfriend, Christian Patrick, 25, were arrested and charged with murder and related offenses.
Contact Robert Moran at 215-854-5983 or firstname.lastname@example.org, or follow @RobertMoran215 on Twitter.