Help!!! Changing Ethernet chipset, do I have to write MAC and PHY driver both?

Guys:

On ref design Intel MAC device and different PHY dev provided, but we
are changing Intel MAC and other PHY devices to integrated MAC + Phy
device, do I have to write MAC driver and PHy driver both?. What is
the diff between MAC and PHY?, why can't it only one?. Is both drivers
are separate or only one.

Thanks in advance. Appreciated for help.