With the game tied 4-4 in the ninth, Shin-Soo Choo (two hits, three RBI) drew a lead-off walk against Mitchell Boggs (0-1) and went to second on a wild pitch.
After Joey Votto was intentionally passed, Choo came home on Phillips' poke that bounced off the right-field foul line.
That set off an outburst in which the Reds sent 15 men to the plate, collecting six hits en route to nine runs.
Jay Bruce finished 4-for-5 with an RBI while Phillips added a solo homer for Cincinnati, which embarrassed St. Louis in its home opener.
Mat Latos (1-0) allowed four runs (only one earned) on four hits over six full frames to notch the win. The right-hander yielded one walk and struck out five.
Yadier Molina collected the only RBI for the Cardinals.