'class WiFiServer' has no member named 'hasHeader'

Hi! I am trying to add a login page to my project. It consists of a button that turns on a led when pressed. The problem is that I get 'class WiFiServer' has no member named 'hasHeader'. I can run the "SimpleAuthentification" project from the examples. I am using an ESP8266-01. Any ideas about how can I solve this?
Here is my code:

#include <ESP8266WiFi.h>
#include <WiFiUdp.h>
#include <NTPClient.h>
#include <WiFiClient.h>
#include <ESP8266WebServer.h>




const long utcOffsetInSeconds = 3*3600;
char daysOfTheWeek[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
WiFiUDP ntpUDP;
NTPClient timeClient(ntpUDP, "pool.ntp.org", utcOffsetInSeconds);

]
const char* ssid     = "ssid";
const char* password = "password";

// Set web server port number to 80
IPAddress ip(127,0,0,53);
IPAddress gateway(192, 168, 0, 1);
IPAddress subnet(255, 255, 255, 0);
WiFiServer server(80);


//Verifica daca e autentificat
bool is_authenticated() {
  Serial.println("Enter is_authenticated");
  if (server.hasHeader("Cookie")) {
    Serial.print("Found cookie: ");
    String cookie = server.header("Cookie");
    Serial.println(cookie);
    if (cookie.indexOf("ESPSESSIONID=1") != -1) {
      Serial.println("Authentication Successful");
      return true;
    }
  }
  Serial.println("Authentication Failed");
  return false;
}

//pagina de logare
void handleLogin() {
  String msg;
  if (server.hasHeader("Cookie")) {
    Serial.print("Found cookie: ");
    String cookie = server.header("Cookie");
    Serial.println(cookie);
  }
  if (server.hasArg("DISCONNECT")) {
    Serial.println("Disconnection");
    server.sendHeader("Location", "/login");
    server.sendHeader("Cache-Control", "no-cache");
    server.sendHeader("Set-Cookie", "ESPSESSIONID=0");
    server.send(301);
    return;
  }
  if (server.hasArg("PASSWORD")) {
    if (server.arg("PASSWORD") == "Ab") {
      server.sendHeader("Location", "/");
      server.sendHeader("Cache-Control", "no-cache");
      server.sendHeader("Set-Cookie", "ESPSESSIONID=1");
      server.send(301);
      //Serial.println("Log in Successful");
      return;
    }
    msg = "Wrong username/password! try again.";
    Serial.println("Log in Failed");
  }
  String content = "<html><body><form action='/login' method='POST'>To log in, please use : admin/admin
";
  content += "Password:<input type='password' name='PASSWORD' placeholder='password'>
";
  content += "<input type='submit' name='SUBMIT' value='Submit'></form>" + msg + "
";
  server.send(200, "text/html", content);
}

void handleRoot() {
  Serial.println("Enter handleRoot");
  String header;
  if (!is_authenticated()) {
    server.sendHeader("Location", "/login");
    server.sendHeader("Cache-Control", "no-cache");
    server.send(301);
    return;
  }
  String content = "<html><body><H2>hello, you successfully connected to esp8266!</H2>
";
  if (server.hasHeader("User-Agent")) {
    content += "the user agent used is : " + server.header("User-Agent") + "

";
  }
  server.send(200, "text/html", content);
}




// Variable to store the HTTP request
String header;

// Auxiliar variables to store the current output state
String output5State = "off";
String output4State = "off";

// Assign output variables to GPIO pins
const int output5 = 2;
//const int output4 = 2;

]
unsigned long currentTime = millis();
// Previous time
unsigned long previousTime = 0; 
]
const long timeoutTime = 2000;

void setup() {
  Serial.begin(115200);
  // Initialize the output variables as outputs
  pinMode(output5, OUTPUT);
  //pinMode(output4, OUTPUT);
  // Set outputs to LOW
  digitalWrite(output5, LOW);
  //digitalWrite(output4, LOW);

  // Connect to Wi-Fi network with SSID and password
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.config(ip, gateway, subnet);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  // Print local IP address and start web server
  Serial.println("");
  Serial.println("WiFi connected.");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());

  server.on("/", handleRoot);
  server.on("/login", handleLogin);
  const char * headerkeys[] = {"User-Agent", "Cookie"} ;
  size_t headerkeyssize = sizeof(headerkeys) / sizeof(char*);
  //ask server to track these headers
  server.collectHeaders(headerkeys, headerkeyssize);
  server.begin();

}

void loop(){
   timeClient.update();

  Serial.print(daysOfTheWeek[timeClient.getDay()]);
  Serial.print(", ");
  Serial.print(timeClient.getHours());
  Serial.print(":");
  Serial.print(timeClient.getMinutes());
  Serial.print(":");
  Serial.println(timeClient.getSeconds());


  
  WiFiClient client = server.available();   // Listen for incoming clients

  if (client) {                             // If a new client connects,

    Serial.println("New Client.");          // print a message out in the serial port
    String currentLine = "";                // make a String to hold incoming data from the client
    currentTime = millis();
    previousTime = currentTime;
    while (client.connected() && currentTime - previousTime <= timeoutTime) { // loop while the client's connected
      currentTime = millis();         
      if (client.available()) {            
        char c = client.read();             
        Serial.write(c);                    
        header += c;
        if (c == '\n') {                    // if the byte is a newline character

          if (currentLine.length() == 0) {
            client.println("HTTP/1.1 200 OK");
            client.println("Content-type:text/html");
            client.println("Connection: close");
            client.println();
            
            // turns the GPIOs on and off
            if (header.indexOf("GET /5/on") >= 0) {
              Serial.println("GPIO 5 on");
              output5State = "on";
              digitalWrite(output5, HIGH);
            } else if (header.indexOf("GET /5/off") >= 0) {
              Serial.println("GPIO 5 off");
              output5State = "off";
              digitalWrite(output5, LOW);
            } 
  
            client.println("<!DOCTYPE html><html>");
            client.println("<head><meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">");
            client.println("<link rel=\"icon\" href=\"data:,\">");

            client.println("<style>html { font-family: Helvetica; display: inline-block; margin: 0px auto; text-align: center;}");
            client.println(".button { background-color: #195B6A; border: none; color: white; padding: 16px 40px;");
            client.println("text-decoration: none; font-size: 30px; margin: 2px; cursor: pointer;}");
            client.println(".button2 {background-color: #77878A;}</style></head>");
            
            // Web Page Heading
            client.println("<body><h1>ESP8266 Web Server</h1>");
            
            // Display current state, and ON/OFF buttons for GPIO 5  
            client.println("<p>GPIO 5 - State " + output5State + "</p>");
            // If the output5State is off, it displays the ON button       
            if (output5State=="off") {
              client.println("<p><a href=\"/5/on\"><button class=\"button\">ON</button></a></p>");
            } else {
              client.println("<p><a href=\"/5/off\"><button class=\"button button2\">OFF</button></a></p>");
            } 
               
            
            client.println("</body></html>");
            
            // The HTTP response ends with another blank line
            client.println();
            // Break out of the while loop
            break;
          } else { // if you got a newline, then clear currentLine
            currentLine = "";
          }
        } else if (c != '\r') {  
          currentLine += c;      
        }
      }
    }

    header = "";

    client.stop();
    Serial.println("Client disconnected.");
    Serial.println("");
  }
}

The problem is that I get 'class WiFiServer' has no member named 'hasHeader'.

Why do you think it does (or should)?