You should set the PORT to 0xFF, otherwise the internal transistors in the mcu are shorted to ground. When you set the port to 0xFF all the pins are helt high by pull-up resistors.
That means that you can set a HIGH pin on the MCU LOW but you can NOT set a LOW pin to HIGH..

If you write 0x00 to the port with your program you will always read a 0x00 value even if you put 5V+ on the pin, you will burn out the port if you do this..