Sir Nick thank you for the clarification.
I was implementing the use of Ethernet shield to send the rfid tag to a php file then pass in to the database so I am not going to use the SD of the ethernet shield, but thanks for the advice I will take note of that.
Sir please correct me If I am wrong, is it the SPI use of both peripherals(ethernet shield w5100 and rfid MFRC522) the problem?
If so, what kind of action should I take? I am very sorry, I just started using arduino recently and I still have little knowledge on it.