IIRC the Philistines were an offshoot of Greeks who had moved to the land of Canaan. Through their interaction with both the people groups of Canaan and with the Greeks, they taught Greece (and thus Rome and Europe) the concept of a written language based on sounds (phonetic language). Thus by days of the Roman Empire, the Philistines became synonymous with the land of Canaan in general. And the people who had been there before became known as the Phonecians.
Well, it’s a pretty good speculation they were Greeks, but it’s not 100% known. They were one of the “Sea Peoples” whose exact origin is still subject to debate. But probably that they were Greek, likely warriors who went on to live as pirates and mercenaries after the Trojan War seems their most likely origin.