#include <Ethernet.h>
#include <SPI.h>
#include <SdFat.h>
#define Ethernet_ss_pin 53
#define Sd_ss_pin 4

// On the Ethernet Shield, CS is pin 4. SdFat handles setting SS
/////////////////////////////////////////////////////////////////////////////////
////////SD Config
/////////////////////////////////////////////////////////////////////////////////

SdFat sd;
SdFile myFile;
//const int chipSelect = 4;
const char* fileName="database.txt";

/////////////////////////////////////////////////////////////////////////////////
////////Ethernet Config
/////////////////////////////////////////////////////////////////////////////////
//String server = "rfid.unisec-bd.org"; //ip Address of the server you will connect to

//The location to go to on the server
//make sure to keep HTTP/1.0 at the end, this is telling it what type of file it is
String location = "/getData.php HTTP/1.0";


// if need to change the MAC address (Very Rare)
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
////////////////////////////////////////////////////////////////////////

EthernetClient client;

char inString[3200]; // string for incoming serial data
int stringPos = 0; // string index counter
boolean startRead = false; // is reading?


void setup() {
  Serial.begin(9600);
  pinMode(53,OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(10,OUTPUT);
  ChipSelect(0);
  Ethernet.begin(mac);
  String pageValue;
  pageValue=connectAndRead(); //connect to the server and read the output
  Serial.println(pageValue); //print out the findings.
  ChipSelect(1);
  // Initialize SdFat or print a detailed error message and halt
  // Use half speed like the native library.
  // change to SPI_FULL_SPEED for more performance.
  if (!sd.begin(Sd_ss_pin, SPI_HALF_SPEED)) {
    sd.initErrorHalt("Sd initialization failed");
  }else{
    Serial.println("Sd card Initialized");
  }
  ChipSelect(0);

}

void loop() {
  Serial.println("Type any character to start");
  while (Serial.read() <= 0) {}
  delay(1000);  // catch Due reset problem
  ChipSelect(2);
  String pageValue = connectAndRead(); //connect to the server and read the output
  Serial.println(pageValue); //print out the findings.
  ChipSelect(1);
  writeSd(pageValue);
  readSd();
  ChipSelect(0);
}
void readSd(){
  //const char path=(const char)fileName;
  if (!myFile.open(fileName, O_READ)) {
    sd.errorHalt("file read error");
  }
  Serial.println(fileName);
  // read from the file until there's nothing else in it:
  char data;
  int c=0;
  while ((data = myFile.read()) >= 0) {
    data=char(data);
    Serial.print(data);
    //Serial.println(c);
    c++;
  }
  // close the file:
  myFile.close();
}

void writeSd(String storeData){
  // open the file for write at end like the Native SD library
  if (!myFile.open(fileName, O_RDWR | O_CREAT | O_AT_END)) {
    sd.errorHalt("opening database.txt for write failed");
  }
  // if the file opened okay, write to it:
  Serial.print("Writing to database.txt...");
  myFile.println(storeData);

  // close the file:
  myFile.close();
  Serial.println("done.");
}

String connectAndRead(){
   ///////// Pin 10,11,12,13 are used as SPI Bus of the ethernet Shield
/// MOSI-         pin 11
/// MISO-         pin 12
/// CLK-          pin 13
/// CS for sdCard-pin 4
/// ss-           pin 10
  // On the Ethernet Shield, CS is pin 4. It's set as an output by default.
  // Note that even if it's not used as the CS pin, the hardware SS pin
  // (10 on most Arduino boards, 53 on the Mega) must be left as an output
  // or the SD library functions will not work.
        //digitalWrite(Sd_ss_pin,HIGH); //SD is not active

 /*       USE THIS CODE FOR PROXY SERVER IN BRAC UNIVERSITY
        // if you get a connection, report back via serial:
  if (client.connect("192.168.1.5", 8080)) {  // This is connecting to the proxy
    Serial.println("connected");
    // Make a HTTP request through proxy:
    client.println("GET rfid.unisec-bd.org/getData.php HTTP/1.0");
    client.println();
  }
*/
  //connect to the server
  Serial.println("connecting...");
  while(!client){;}
  //port 80 is typical of a www page
  if (client.connect("rfid.unisec-bd.org", 80)) {
    Serial.println("connected");
    client.print("GET ");
    client.println(location);
    client.println("Host: rfid.unisec-bd.org"); // SERVER ADDRESS HERE TOO
    client.println("Content-Type: application/x-www-form-urlencoded");
    client.println();
    //Connected - Read the page
    return readPage(); //go and read the output
  }else{
    return "connection failed";
  }
}

String readPage(){
  //read the page, and capture & return everything between '<' and '>'
  stringPos = 0;
  memset( &inString, 0, 3200 ); //clear inString memory
  while(true){
    if (client.available()) {
      char c = client.read();
      if (c == '!' ) { //'!' is our begining character
        startRead = true; //Ready to start reading the part
      }else if(startRead){
        if(c != '@'){ //'@' is our ending character
          inString[stringPos] = c;
          stringPos ++;
        }else{
          //got what we need here! We can disconnect now
          startRead = false;
          client.stop();
          client.flush();
          Serial.println("disconnecting.");
          return inString;
        }
      }
    }
  }
}
void ChipSelect(int s){
  if(s=0){                              //Both Device is off
    //digitalWrite(53,HIGH);
    digitalWrite(10,HIGH);
    digitalWrite(4,HIGH);
  }else if(s=1){                        //Only SD is active
    //digitalWrite(53,HIGH);
    digitalWrite(10,HIGH);
    digitalWrite(4,LOW);
  }else if(s=2){                        //Only Ethernet is Active
    //digitalWrite(53,HIGH);
    digitalWrite(10,LOW);
    digitalWrite(4,HIGH);
  }
  Serial.print("Ethernet SS:");
  Serial.println(digitalRead(10));
  Serial.print("SD SS:");
  Serial.println(digitalRead(4));
}

https://stackoverflow.com/questions/41277010/how-to-connect-rc522-rfid-module-and-sd-card-adapter-to-one-arduino-board/46797716#46797716

`//HEADER FILES##################################
#include <SPI.h>
#include <MFRC522.h>
#include <SoftwareSerial.h>
#include <EEPROM.h>
#include <SD.h>
//ARDUINO MEGA
//MISO 50
//MOSI 51
//SCK 52
//ON/OFF SD CARD READER 2
//SD Card DEfinitions###########################
File root;
const int chipSelect = 4;
const int chipONOFF = 2;
//RF_ID Definitions#############################
#define SS_PIN 7
#define RST_PIN 6
MFRC522 mfrc522(SS_PIN, RST_PIN);

void setup()
{
Serial.begin(9600);
SPI.begin();
pinMode(chipONOFF,OUTPUT);
digitalWrite(chipONOFF, LOW);
tc_03_rfIDbasicTest();
digitalWrite(chipONOFF, HIGH);
tc_02_SDcardConnectTest();
digitalWrite(chipONOFF, LOW);
tc_03_rfIDbasicTest();
}

void loop()
{ }

void tc_02_SDcardConnectTest()
{
Serial.println("");
Serial.println("");
Serial.println("-----TEST 2: FUNCIONAMIENTO BASICO DE SD-----");
Serial.println("");
if (!SD.begin(chipSelect))
{
Serial.println("Falla en comunicacion, asegurate que la uSD esté insertada e intenta de nuevo");
Serial.println("FAIL");
}
else
{
Serial.println("Tarjeta uSD Detectada");
Serial.println("PASS");
root = SD.open("/");
printDirectory(root, 0);
}
}

int tc_03_rfIDbasicTest()
{
Serial.println("-----TEST 3: FUNCIONAMIENTO BASICO DE RFid-----");
mfrc522.PCD_Init(); // Init MFRC522 chip again per each write attemp
mfrc522.PCD_DumpVersionToSerial(); // Show details of PCD - MFRC522 Card Reader details

if ( ! mfrc522.PICC_IsNewCardPresent())
    Serial.println("Targeta fuera de alcance");
else
    Serial.println("Targeta dentro de alcance");
if ( ! mfrc522.PICC_ReadCardSerial())
    Serial.println("Falla en lectura de tarjeta");
else
{
    Serial.println("RFid OK :) !");
    Serial.println("PASS");
    String nuid="";
    for (byte i = 0; i < 4; i++)
        nuid+=mfrc522.uid.uidByte[i];
    mfrc522.PICC_HaltA();
    mfrc522.PCD_StopCrypto1();
    Serial.println(nuid);
}
}

void printDirectory(File dir, int numTabs)
{
while (true)
{
File entry = dir.openNextFile();
if (! entry)
break;
for (uint8_t i = 0; i < numTabs; i++)
{
Serial.print('t');
}
Serial.print(entry.name());
if (entry.isDirectory())
{
Serial.println("/");
printDirectory(entry, numTabs + 1);
}
else
{
Serial.print("tt");
Serial.println(entry.size(), DEC);
}
entry.close();
}
}`

`-----TEST 3: FUNCIONAMIENTO BASICO DE RFid-----
Firmware Version: 0x92 = v2.0
Targeta dentro de alcance
RFid OK :) !
PASS
215103959

-----TEST 2: FUNCIONAMIENTO BASICO DE SD-----

Tarjeta uSD Detectada
PASS
BIO/
D70A273B.DB 44
ENROLADO.DB 57
F683B22D.DB 40
A6DE1E49.DB 38
080732DC.DB 38
08D1EF01.DB 38
08267788.DB 38
XWIFI.DB    47
CP0101.DB   26
XFEC.DB 4
CP0102.DB   26
CP0201.DB   26
SYSTEM1/
WPSETT1.DAT 12
INDEXE~1    76
CP0301.DB   26
GP01.DB 33
GP02.DB 33
GP03.DB 33
GP04.DB 32
GP05.DB 32
GS06.DB 35
GS07.DB 37
GS08.DB 36
GS09.DB 36
GS10.DB 36
GE03.DB 24
GE02.DB 20
GE01.DB 23
GS11.DB 38
AP010101.DB 69
AP010102.DB 83

-----TEST 3: FUNCIONAMIENTO BASICO DE RFid-----
Firmware Version: 0x92 = v2.0
Targeta dentro de alcance
RFid OK :) !
PASS
215103959`