Untitled

<?php
    session_start();

    $db = mysqli_connect("localhost","root", "", "authentication");

    if (isset($_POST["register_btn"])) {
        session_start();

        $username = mysql_real_escape_string($_POST["username"]);
        $email = mysql_real_escape_string($_POST["email"]);
        $password = mysql_real_escape_string($_POST["password"]);
        $password2 = mysql_real_escape_string($_POST["password2"]);

        if ($password == $password2) {
            $password = md5($password);
            $sql = "INSERT INTO users(username,email,password) VALUES($username","$email","$password")";
            mysqli_query($db,$sql);
            $_SESSION["message"] = $username;
        }else{
            $_SESSION["message"] = "Password dont match";
        }
    }
?>


<!DOCTYPE html>
<html>
<head>
    <head>
    <title>Some Text - Login</title>
    <meta charset="UTF-8">
    <!--CSS-->
    <link rel="stylesheet" type="text/css" href="https://www.w3schools.com/w3css/4/w3.css">
    <link rel="stylesheet" type="text/css" href="styles.css">
    <link rel="stylesheet" type="text/css" href="hoveranim.css">
    <!--Awesome Icons-->
    <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.4.1/css/all.css" integrity="sha384-5sAR7xN1Nv6T6+dT2mhtzEpVJvfS3NScPQTrOxhwjIuvcA67KV2R5Jz6kr4abQsz" crossorigin="anonymous">
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
    <!--Fonty-->
    <link href="https://fonts.googleapis.com/css?family=Charmonman" rel="stylesheet">
    <link href="https://fonts.googleapis.com/css?family=Fanwood+Text" rel="stylesheet">
</head>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>

<form method="post" action="register.php">
    <table>
        <tr>
            <td>username</td>
            <td><input type="text" name="username" class="textInput"></td>
        </tr>
        <tr>
            <td>username</td>
            <td><input type="email" name="email" class="textInput"></td>
        </tr>
        <tr>
            <td>username</td>
            <td><input type="password" name="password" class="textInput"></td>
        </tr>
        <tr>
            <td>username</td>
            <td><input type="password" name="password2" class="textInput"></td>
        </tr>
        <tr>
            <td>username</td>
            <td><input type="sumbit" name="register_btn" value="Register"></td>
        </tr>
    </table>
</form>

  <div class="container signin">
    <p>Already have an account? <a href="login.html">Sign in</a>.</p>
  </div>
</form>

</body>
</html>