The best places to obtain autographs from Phillies players are shown on this page.
For a more detailed overview, read the Getting autographs section on the Spectrum Field guide page.
Section 120, shown below, is the best one to be in, as players often start signing there and don't always make their way down to 118.
The best place to be is actually at the bottom of section P1 in the Picnic Terrace, but that's a groups-only area that becomes restricted more than an hour before game time.
The reason why the left field line is the place to get autographs is seen below: the opening in the green padding is the player entrance to their clubhouse.
Outside the stadium after the game, patient fans waiting aside the fence that separtes them from the players' parking lot are often rewarded by Phillies players on their way to pick up their keys from the valet stand.
Autograph success seen at the black wrought iron fence that most players walk close to in the parking lot, which is near the 3rd base entrance (West gate) to Bright House Field.