login and signup

//This is for registration.php
<?php include('server.php'); ?>
<!DOCTYPE html>
<html>
<head>
    <title>Registration</title>
    <link rel="stylesheet" type="text/css" href="./css/styles.css">
</head>
<body>
    <center>
        <div id="signup_form_center" style="margin: 70px 0 50px;">
            <img src="./logo/wecon_logo_single.png" height="50px" viewBox="-3 -3 82 82" width="50">
            <h1 style="text-align: center; font-weight: bold; ">Sign Up</h1>
            <form action="registration.php" method="post">
                <!-- display validation errors here -->
                <?php include('errors.php') ?>
                <input type="text" name="username" value="<?php echo $username; ?>"placeholder="Username">
                <br>
                <input type="email" name="email" value="<?php echo $email; ?>"placeholder="Email Address">
                <br>
                <input type="password" name="password_1" required="required"placeholder="Password">
                <br>
                <input type="password" name="password_2" required="required"placeholder="Confirm Password">
                <br>
                <center>
                    <input type="submit" name="reg_user" class="button">
                </center>
            </form>
            <i>Already a user <a href="login.php" class="signup_a">Login</a></i>
        </div>
    </center>
</body>
</html>
//registration.php end


//This is for login.php
<?php include('server.php') ?>
<!DOCTYPE html>
<html>
<head>
    <title>Log in</title>
    <link rel="stylesheet" type="text/css" href="./css/styles.css">
    <link rel="stylesheet" type="text/css" href="./css/w3.css">
</head>
<body>
    <center>
        <div id="login_form_center">
            <img src="./logo/wecon_logo_single.png" height="50px" viewBox="-3 -3 82 82" width="50">
            <h1 style="text-align: center; font-weight: bold; ">The Right Place For Business</h1>
            <div style="margin: 0 auto; width: 268px;">
                <form action="login.php" method="post">
                    <!-- display validation errors here -->
                    <?php include('errors.php'); ?>
                    <br>
                    <input type="text" name="username" placeholder="Username" required="required" style="height: 40px;width: auto";>
                    <br>
                    <input type="password" name="password" placeholder="Password" required="required" style="height: 40px;width: auto">
                    <br>
                    <button type="submit" name="login" class="button"> Login </button>
                </form>
            </div>
            <i>Please if you don't have an account <a onclick="document.getElementById('id01').style.display='block'" class="signup_a">Sign Up</a>
            </i>
        </div>
    </center>
</body>
</html>
//login.php end

//This is for server.php
<?php
    session_start();

    $username = "";
    $email = "";
    $error = array();

    //connect to db
    $db = mysqli_connect('localhost', 'root', '', 'practice');

    // if the register button is clicked
    if (isset($_POST['reg_user'])) {
        $username = mysqli_real_escape_string($db, $_POST['username']);
        $email = mysqli_real_escape_string($db, $_POST['email']);
        $password_1 = mysqli_real_escape_string($db, $_POST['password_1']);
        $password_2 = mysqli_real_escape_string($db, $_POST['password_2']);

    // ensure that form fields are filled properly
    if (empty($username)) {
        array_push($errors, "Username is required");
        }
    if (empty($email)) {
        array_push($errors, "Email is required");
        }
    if (empty($password)) {
        array_push($errors, "Password is required");
        }
    if ($password_1 != $password_2) {
        array_push($errors, "Passwords do not match");
        }

        //if there are no errors, save user to database
    if (count($errors) == 0) {
        $password = md5($password_1); //This will encrypt the password
        $sql = "INSERT INTO user (username, email, password)
                    VALUES ('$username', '$email', '$password')";
        mysqli_query($db, $sql);
        $_SESSION['username'] = $username;
        $_SESSION['success'] = "You are now logged in";
        header('location: index.php'); //redirect to home page
    }

    }


    // log user in from login page
    if(isset($_POST['login'])) {
        $username = mysqli_real_escape_string($db, $_POST['username']);
        $password = mysqli_real_escape_string($db, $_POST['password']);

        // ensure that form fields are filled properly
        if (empty($username)) {
            array_push($error, "Username is required");
        }
        if (empty($password)) {
            array_push($error, "Password is required");
        }
        if (count($error) == 0 ) {
            $passord = md5($password); //encrypt password before comparing with that from database
            $query = "SELECT * FROM user WHERE username='$username' AND password='$password'";
            $result = mysqli_query($db, $query);
            if (mysqli_num_rows($result) == 1) {
                // log user in
                $_SESSION['username'] = $username;
                $_SESSION['success'] = "You are now logged in";
                header("location: index.php"); //redirect to homepage
            }else{
                array_push($error, "Wrong username/password combination. Please try again.");
            }
        }
    }


// logout
     if (isset($_GET['logout'])) {
        session_destroy();
        unset($_SESSION['username']);
        header('location: login.php');
     }

//This is the end of server.php

//This is for index.php
<?php include('server.php');

     // if user is not logged in, they cannot access this page
     if (empty($_SESSION['username'])) {
        header('location: login.php');
     }
?>

<!DOCTYPE html>
<html>
<head>
    <title>Home Page</title>
    <link rel="stylesheet" type="text/css" href="/css/style.css">
</head>
<body>

    <h1>This is the homepage</h1>
    <?php if(isset($_SESSION['success'])): ?>
          <div>
            <h3>
                <?php
                    echo $_SESSION['success'];
                    unset($_SESSION['success']);
                 ?>
            </h3>
        </div>
    <?php endif ?>

    <?php
        // if the user logs in print information about him
        if (isset($_SESSION["username"])) : ?>
        <h3>Welcome <strong><?php echo $_SESSION['username']; ?> ?></strong></h3>
        <p><a href="index.php?logout='1'" style="color: red;">Logout</a></p>

     <?php endif ?>


</body>
</html>

//end of index.php


//This is for errors.php
<?php if (count($error) > 0 ): ?>
    <div>
        <?php foreach ($error as $error): ?>
            <p><?php echo $error; ?></p>
        <?php endforeach ?>
    </div>
<?php endif ?>

//End of errors.php