Let's strip the code a little bit to make it more readable:

#include <SoftwareSerial.h>
#include <SPI.h>
#include <Ethernet.h>
 
SoftwareSerial rfidSerial(2, 3); // (RX, TX)
String myTag, myTagClean;
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte ip[] = {10, 1, 1, 2}; // ip in lan assigned to arduino
//byte gateway[] = {10, 1, 1, 254 }; // internet access via router
byte subnet[] = {255, 255, 255, 0 }; //subnet mask
char server[] = "10.1.1.1";
EthernetClient client;
 
void setup() {
  Serial.begin(9600);
  Serial.println("Hello World : RFID reader, from hardware serial");
  
  rfidSerial.begin(9600); // communication speed of ID-12 reader
  rfidSerial.println("Hello World : RFID reader, from software serial");
}
void loop(){
  if (rfidSerial.available()) {
   char incoming = (char)rfidSerial.read(); 
 
   if(incoming == 3){
 
      Serial.println("Incoming Tag");
 
      myTagClean = myTag.substring(1, 13);
      myTag = "";      
      
      Serial.println(myTagClean);
 
      if(myTagClean == "13004C8ED302"){
        Serial.println("White Card"); 
      }
      else if(myTagClean == "01023C245D46"){
        Serial.println("Keychain"); 
      }
 
    }
    else{
      myTag = String (myTag + incoming);
    }
 
  }
  
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    Ethernet.begin(mac, ip);
  }
  // give the Ethernet shield a second to initialize:
  delay(1000);
  Serial.println("connecting...");
  Serial.println("connected");
  
  // if you get a connection, report back via serial:
  if (client.connect(server, 80)) {
   
    // Make a HTTP request:
    client.print("GET /test/add_data.php?");
    client.println("myTagClean= ");
    client.println(myTagClean);
    client.println( " HTTP/1.1");
    client.print( "Host: " );
    client.println(server);
    client.println( "Connection: close" );
    client.println();
    client.println();
    client.stop();
  } 
  else {
    Serial.println("connection failed");
  }
}

First things I notice:

1) You commented out the network Gateway. Not sure if things will work when you do that.

2) I do not know where rfidSerial is defined, is that from the SoftwareSerial library?

OK, next step: what is happening (or not happening).

Do you get the RFID to work at all? I mean do you get output on the serial monitor?

Thanks for moving it to the a new topic

1) I'm not sure what do you mean by commenting out of the gateway?

2) It is defined from the SoftwareSerial library. This is where I refer https://www.arduino.cc/en/Tutorial/SoftwareSerialExample

After uploading, the serial monitor shows that it's connected. When I tried to using the RFID tag on the reader, the content of the tag didn't show up. 

No problem ...

Well in your code it says:

//byte gateway[] = {10, 1, 1, 254 }; // internet access via router

which should probably be

byte gateway[] = {10, 1, 1, 254 }; // internet access via router

As for the problem you're running into: so the RFID reading part is not working, or not producing any data.
Do you have working example code?

Normally I'd make sure that at least each item individually works.

So one sketch for RFID reading, to make sure our hardware works.
And one for networking, so we know the network part works.

After both work, we should see how to merge them.
I do however have no experience with RFID readers - I might have one laying around, but I'd have to look for that one.

What mode RFID reader/cards do you have?

I meant: what kind of RFID do you have?

Actually this line is not in used...

//byte gateway[] = {10, 1, 1, 254 }; // internet access via router

The code below is what I used for reading the RFID tag.

#include <SoftwareSerial.h>
SoftwareSerial rfidSerial(2, 3); // (RX, TX)
String myTag, myTagClean;
 
void setup(){
  Serial.begin(9600);
  Serial.println("Starting Staff Attendance Using RFID...");
 
  // set the data rate for the SoftwareSerial port
  rfidSerial.begin(9600); // communication speed of ID-12 reader
  rfidSerial.println("Starting Staff Attendance Using RFID...");
}
 
void loop(){
 
  if (rfidSerial.available()) {
    char incoming = (char)rfidSerial.read(); 
    if(incoming == 3){
 
      Serial.println("Incoming Tag");
      myTagClean = myTag.substring(1, 13);
 
      // reset myTag
      myTag = "";      
 
      Serial.println(myTagClean);
 
      // test and compare
      if(myTagClean == "13004C8ED302"){
        Serial.println("White Card"); 
      }
 
      if(myTagClean == "01023C245D46"){
        Serial.println("Keychain"); 
      }
 
    }
 
    // standard char, we add it to the String
    else{
      myTag = String (myTag + incoming);
    }
 
  }
 
}

This is the library that I have been using to make a http request to the wampserver:

//Final Year Project - Japan Malaysian Technical Institute (JMTi)
//Title                       : Staff Attendance Using RFID
//Written and developed by    : Mohammad Syafiq bin Yusaini
//Group members               : Tan Soo Xian
//                            : Siew Johnnie
//Example sketch for RFID ID-20LA reader
//===================================================================
/*
 */
#include <SPI.h>
#include <Ethernet.h>
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte ip[] = {10, 1, 1, 2}; // ip in lan assigned to arduino
//byte gateway[] = {10, 1, 1, 254 }; // internet access via router
byte subnet[] = {255, 255, 255, 0 }; //subnet mask
char server[] = "10.1.1.1";
EthernetClient client;
void setup() {
 // Open serial communications and wait for port to open:
  Serial.begin(9600);
   while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }
  // start the Ethernet connection:
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    // no point in carrying on, so do nothing forevermore:
    // try to congifure using IP address instead of DHCP:
    Ethernet.begin(mac, ip);
  }
  // give the Ethernet shield a second to initialize:
  delay(1000);
  Serial.println("connecting...");
  // if you get a connection, report back via serial:
  if (client.connect(server, 80)) {
    Serial.println("connected");
    // Make a HTTP request:
    client.print("GET /test/add_data.php?");
    //client.println(myTag);                 
    client.print("myTag=");
    client.print( "5665656" );
    client.println( " HTTP/1.1");
    client.print( "Host: " );
    client.println(server);
    client.println( "Connection: close" );
    client.println();
    client.println();
    client.stop();
  } 
  else {
    // kf you didn't get a connection to the server:
    Serial.println("connection failed");
  }
}
void loop()
{
  // if there are incoming bytes available 
  // from the server, read them and print them:
  if (client.available()) {
    char c = client.read();
    Serial.print(c);
  }
  // if the server's disconnected, stop the client:
  if (!client.connected()) {
    Serial.println();
    Serial.println("disconnecting.");
    client.stop();
    // do nothing forevermore:
    while(true);
  }
}

I have tried out both of the libraries separately and both of them are working fine. The problem now is to merge of the libraries. 

I'd first start with a clean ethernet sketch, something like the code below, which tries to submit a tag, waits 10 seconds (so we do not keep pushing data repeatedly to the server).

#include <UIPEthernet.h> // Used for Ethernet
// * ETHERNET SETTING *
// Arduino Uno pins: 10 = CS, 11 = MOSI, 12 = MISO, 13 = SCK
// Ethernet MAC address - must be unique on your network - MAC Reads T4A001 in hex (unique in your network)
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
// For the rest we use DHCP (IP address and such)
char server[] = "192.168.1.100"; // IP Adres (or name) of server to dump data to
byte ip[] = {10, 1, 1, 2}; // ip in lan assigned to arduino
byte gateway[] = {10, 1, 1, 254 }; // internet access via router
byte subnet[] = {255, 255, 255, 0 }; //subnet mask
char server[] = "10.1.1.1";
EthernetClient client;
void setup() {
  Serial.begin(9600);
  Ethernet.begin(mac);
  Serial.print("IP Address        : ");
  Serial.println(Ethernet.localIP());
  Serial.print("Subnet Mask       : ");
  Serial.println(Ethernet.subnetMask());
  Serial.print("Default Gateway IP: ");
  Serial.println(Ethernet.gatewayIP());
  Serial.print("DNS Server IP     : ");
  Serial.println(Ethernet.dnsServerIP());
}
void loop() {
  // if you get a connection, report back via serial:
  if (client.connect(server, 80)) {
    Serial.println("-> Connected");
    // Make a HTTP request:
    client.print( "GET /testserver/arduino_temperatures/add_data.php?");
    client.print("myTagClean= ");
    client.print("123456"); // just a test value
    client.println( " HTTP/1.1");
    client.print( "Host: " );
    client.println(server)
    client.println( "Connection: close" );
    client.println();
    client.println();
    client.stop();
  }
  else {
    // you didn't get a connection to the server:
    Serial.println("--> connection failed/n");
  }

  delay(10000); // wait 10 seconds so it doesn't overload the server
}

Just notice a mistake in that code:

client.print( "GET /testserver/arduino_temperatures/add_data.php?");

should be:

client.print("GET /test/add_data.php?");

I also noticed the use of client.println when writing to the ethernet connection, which probably should have been a client.write (see the example I just posted).

Anyhow, next step is to take the RFID example and bring the reading part out of the loop, something like this (I cannot test this). Note that this can be done more efficient, I'm just trying to illustrate how I'd normally would merge two sketches.

I hope I did everything OK with the boolean return value of ReadRFID(). It compiles allright though. (please test)

#include <SoftwareSerial.h>
SoftwareSerial rfidSerial(2, 3); // (RX, TX)
String myTag, myTagClean;
 
void setup(){
  Serial.begin(9600);
  Serial.println("Starting Staff Attendance Using RFID...");
 
  // set the data rate for the SoftwareSerial port
  rfidSerial.begin(9600); // communication speed of ID-12 reader
  rfidSerial.println("Starting Staff Attendance Using RFID...");
}
 
void loop(){
  if(ReadRFID()) {
    if(myTagClean == "13004C8ED302"){
      Serial.println("White Card"); 
    }
    else if(myTagClean == "01023C245D46"){
      Serial.println("Keychain"); 
    }
  }
 
}
boolean ReadRFID() {
  if (rfidSerial.available()) {
    char incoming = (char)rfidSerial.read(); 
    if(incoming == 3){
      Serial.println("ReadRFID: Incoming Tag");
      myTagClean = myTag.substring(1, 13);
      myTag = "";      
      Serial.println(myTagClean);
     return true;
    }
    else{
      myTag = String (myTag + incoming);
      return false;
    }
  }
}

If the function ReadRFID() worked (in the previous example/test), the the code could look something like the code below.

We setup all the Ethernet and RFID stuff, and keep calling the ReafRFID() function (a delay in between might be desired)

(I noticed another mistake in the previous code: remove the first line with "char server" - it's in there twice, which is wrong)

#include <UIPEthernet.h> // Used for Ethernet
#include <SoftwareSerial.h>  // Used for RFID
// ETHERNET
// Arduino Uno pins: 10 = CS, 11 = MOSI, 12 = MISO, 13 = SCK
// Ethernet MAC address - must be unique on your network - MAC Reads T4A001 in hex (unique in your network)
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte ip[] = {10, 1, 1, 2}; // ip in lan assigned to arduino
byte gateway[] = {10, 1, 1, 254 }; // internet access via router
byte subnet[] = {255, 255, 255, 0 }; //subnet mask
char server[] = "10.1.1.1";
EthernetClient client;
// RFID
SoftwareSerial rfidSerial(2, 3); // (RX, TX)
String myTag, myTagClean;
boolean ReadRFID() {
  if (rfidSerial.available()) {
    char incoming = (char)rfidSerial.read(); 
    if(incoming == 3){
      Serial.println("ReadRFID: Incoming Tag");
      myTagClean = myTag.substring(1, 13);
      myTag = "";      
      Serial.println(myTagClean);
    return true;
    }
    else{
      myTag = String (myTag + incoming);
      return false;
    }
  }
}
void setup() {
  Serial.begin(9600);
  Ethernet.begin(mac);
  Serial.print("IP Address        : ");
  Serial.println(Ethernet.localIP());
  Serial.print("Subnet Mask       : ");
  Serial.println(Ethernet.subnetMask());
  Serial.print("Default Gateway IP: ");
  Serial.println(Ethernet.gatewayIP());
  Serial.print("DNS Server IP     : ");
  Serial.println(Ethernet.dnsServerIP());
  
  rfidSerial.begin(9600); // communication speed of ID-12 reader
  rfidSerial.println("Starting Staff Attendance Using RFID...");
}
void loop() {
  // if RFID read an ID ...
  if(ReadRFID()) {
    // ... and if we get a connection, send the RFID code to the server:
    if (client.connect(server, 80)) {
      Serial.println("-> Connected");
      // Make a HTTP request:
      client.print( "GET /test/add_data.php?");
      client.print("myTagClean= ");
      client.print(myTagClean); // <--- Maybe let the server decide to do with that number?
      client.println( " HTTP/1.1");
      client.print( "Host: " );
      client.println(server);
      client.println( "Connection: close" );
      client.println();
      client.println();
      client.stop();
    }
    else {
      // you didn't get a connection to the server:
      Serial.println("--> connection failed/n");
    }
  }
}

Note that I moved the ReadRFID() function to the front, seems required so the compiler knows the function definition properly. This means that it should have failed in the previous example as well,... Like I said: I cannot really test any of this ... 

Also note that gateway definition is probably needed for Ethernet to work properly.

Thanks for helping. I tried the code that had been merged. The RFID tag did show in the serial monitor but it sent an empty input to the database.

<?php
    // Connect to MySQL
    include("conn.inc");
    // Prepare the SQL statement
    $SQL = "INSERT INTO test(id) VALUES ('".$_GET["myTagClean"]."')";     
    // Execute SQL statement
    mysql_query($SQL);
?>

Maybe it's the space between table name (test) and field name (id):

<?php
    // Connect to MySQL
    include("conn.inc");
    // Prepare the SQL statement
    $SQL = "INSERT INTO test (id) VALUES ('".$_GET["myTagClean"]."')";     
    // Execute SQL statement
    mysql_query($SQL);
?>

Thanks. After removing the space made it work.