* @brief  Writes to a PHY register.
  * @param  heth: pointer to a ETH_HandleTypeDef structure that contains
  *         the configuration information for ETHERNET module
  * @param  PHYAddr: PHY port address, must be a value from 0 to 31   
  * @param  PHYReg: PHY register address, must be a value from 0 to 31
  * @param  RegValue: the value to write
  * @retval HAL status

自问自答了。H7每次对PHY进行读写的时候的形参里有一个PHY地址。