Because I am trying to process an immigration work permit for Canada, (I am an American citizen), I need to fly directly into Halifax from the States. I am trying to make sure that I don't clear Canadian immigration and customs in the Philadelphia airport. (For instance, when I fly from Toronto to the USA, I clear American immigration and customs there.) Any ideas?
No, American airports don't offer preclearance the way Canadian ones do. (The reason is the relative number of airports in each country: the preclearance in a small handful of Canadian cities enables flights into hundreds of American cities. To do it the other way around would be crazy.)
You will clear customs and immigration when you land in Halifax.