Now, of course, liberal scholars just assume that since the Bible is one big fantasy, of course the authors of the Bible were just making things up and we can tell this especially here because in reality there were no Philistines at the time of Abraham or the other Patriarchs. One of the difficulties with this explanation is the fact that Abraham looks rather bad in his encounter with the Philistine king, since, yet again, he lies to him about his wife being his sister. So, why would later Hebrew authors make up stories that made their ancestors look bad (actually this is a question for most of the Bible- Gospels included)?
Kitchen points out that it is not necessarily the case that there were no Philistines back at Abraham's time. Philistine probably for the Hebrews is simply a general term for "Sea People" name, the Myceaneans, the forerunners of the Greeks. Now, archaeologically speaking there is considerable evidence that these people were in Palestine at the time of Abraham. There have been Palaces found and also a lot of pottery showing their presence specifically in the area later occupied by the Philistines, as well as in Galilee. Furthermore, the Philistines that Abraham encounters live in one city and not five. They are friendly to him, rather than hostile. So, it is very unlikely that a later Hebrew author is projecting later historical reality back on the time of Abraham.
Ultimately, the presence of the word "Philistines" to describe the sea peoples that the Patriarchs encounter most likely have their origin in the later transcription of the Pentateuch at the time of David. Remember Kitchen makes the suggestion (which I think is correct) that the Pentateuch was originally written in a common western Semitic alphabet and then transcribed at the time of David into Paleo-Hebrew script. At that time, certain aspect of the vocabulary and grammar would have been updated. Being that at the time of David the Sea Peoples were specifically Philistines, what was originally (probably) "Caphtorites" was transcribed into "Philistines."