score:14
If you miss the first leg of your trip, you will be treated as a "no-show" and the rest of that trip will be cancelled.
If you had a flight XYZ-PHL-LGA and wanted to get off at PHL without finishing the journey, that is okay because you don't need the rest of the trip anyway. Most of the time, your seat will be given to someone on standby. If you have checked luggage, it might take some time to retrieve. This is also a good way to lose your checked luggage... :)
Sometimes, sneaky travel agents can actually save you money by making these kind of tickets, where you are booked onto flights that you will never use, just because the airline has a campaign that ends up cheaper than a normal fare. The airlines don't generally like this, and it is always at your own risk.