‘And Abraham sojourned in the land of the Philistines many days.' Notice the stress on the fact that he is a sojourner. Though he has settled down there the land is not his people's, as one day it will be. He still has to walk by faith.
“In the land of the Philistines.” It is clear that the area where they were was acknowledged to be under the control of the Philistine group. This may not be the name of the area but just an acknowledgement of the facts.
“Many days.” The idea of Abraham wandering continually around from place to place is incorrect. Here ‘many days' probably means a number of years. He was there when Isaac was born. He was there when Isaac was a growing lad (Genesis 22). Of course, the flocks and herds had to be moved about to find grazing, but this was done from a permanent centre.