what is the difference between PHY and MAC chip
A PHY chip or layer converts data between a "clean" clocked digital form which is only suitable for very-short-distance (i.e. inches) communication, and an analogue form which is suitable for longer range transmission.  It has no particular clue as to what any of the bits "mean", nor how they should be interpreted or assembled.  The MAC chip or layer receives bits from the PHY, detects packet boundaries, assembles bits into packets, and validates them.  It also takes packets of data that are loaded into it and converts them to streams of bits which are fed to the PHY.  Typically, a MAC will include some logic to delay transmissions until the line is clear, and retry transmissions which are interrupted by collisions, but it will not include logic to listen for acknowledgments nor retry packets which are not garbled by collisions but aren't acknowledged either.