Welcome <?php echo $user["name"]; ?>

function.php///////////


&lt;?php
session_start();
$conn = mysqli_connect("localhost", "root", "", "loginregister");

// IF
if(isset($_POST["action"])){
  if($_POST["action"] == "register"){
    register();
  }
  else if($_POST["action"] == "login"){
    login();
  }
}

// REGISTER
function register(){
  global $conn;

  $name = $_POST["name"];
  $username = $_POST["username"];
  $password = $_POST["password"];

  if(empty($name) || empty($username) || empty($password)){
    echo "Please Fill Out The Form!";
    exit;
  }

  $user = mysqli_query($conn, "SELECT * FROM tb_user WHERE username = '$username'");
  if(mysqli_num_rows($user) > 0){
    echo "Username Has Already Taken";
    exit;
  }

  $query = "INSERT INTO tb_user VALUES('', '$name', '$username', '$password')";
  mysqli_query($conn, $query);
  echo "Registration Successful";
}

// LOGIN
function login(){
  global $conn;

  $username = $_POST["username"];
  $password = $_POST["password"];

  $user = mysqli_query($conn, "SELECT * FROM tb_user WHERE username = '$username'");

  if(mysqli_num_rows($user) > 0){

    $row = mysqli_fetch_assoc($user);

    if($password == $row['password']){
      echo "Login Successful";
      $_SESSION["login"] = true;
      $_SESSION["id"] = $row["id"];
    }
    else{
      echo "Wrong Password";
      exit;
    }
  }
  else{
    echo "User Not Registered";
    exit;
  }
}
?&gt;

index.php/////////


&lt;?php
require 'function.php';
if(isset($_SESSION["id"])){
  $id = $_SESSION["id"];
  $user = mysqli_fetch_assoc(mysqli_query($conn, "SELECT * FROM tb_user WHERE id = $id"));
}
else{
  header("Location: login.php");
}
?&gt;
<!DOCTYPE html>
&lt;html lang="en" dir="ltr"&gt;
  &lt;head&gt;
    &lt;meta charset="utf-8"&gt;
    &lt;title&gt;Index&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
    <h1>Welcome &lt;?php echo $user["name"]; ?&gt;</h1>
    <a href="logout.php">Logout</a>
  &lt;/body&gt;
&lt;/html&gt;