PHP - $ _SESSION问题

I am creating an application using PHP and I am trying to pass information using $_SESSION superglobal, but for some reason on one of the pages $_SESSION is not working. I have a registration form, a login form and an admin page. When the user register successfully he will be redirected to the login page to add his credentials. When the user add his credentials he will redirected to the admin page. In the admin page I have a welcome message. The message is <?php echo "Welcome ". $_SESSION['name']; ?>

Moreover, when the user goes to the admin page the message is empty. It does not read the $_SESSION['name']. I have print_r($_SESSION) but the array is empty.

Can someone tell me what I am doing wrong?

Thanks.

CODE:

Registration Form:

<?php include "includes/header.php"; ?>
<?php include "includes/navigation.php"; ?>
<?php session_start(); ?>


<div class="container">

    <div class="row">

        <div class="col-xs-6 col-xs-offset-3">
            <div class="well">
                <h4>Registration Form</h4>
                <form method="post" action="">

                    <?php

                        if(isset($_POST['register'])) {

                            $name = mysqli_real_escape_string($connection, $_POST['firstname']);
                            $lastname = mysqli_real_escape_string($connection, $_POST['lastname']);
                            $email = mysqli_real_escape_string($connection, $_POST['email']);

                            if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
                              echo("$email is not a valid email address<br>");
                            } 

                            $password = mysqli_real_escape_string($connection, $_POST['password']);
                            $dob = mysqli_real_escape_string($connection, $_POST['dob']);
                            $gender = mysqli_real_escape_string($connection, $_POST['gender']);

                            $password = password_hash($password, PASSWORD_BCRYPT);

                            $check_if_user_exist = "SELECT email FROM users WHERE email = '$email' ";

                            $check_if_user_exist_result = mysqli_query($connection, $check_if_user_exist);

                            if(mysqli_num_rows($check_if_user_exist_result) >= 1) {

                                echo "User Exist... <br>Please try another email";

                            }else {


                            $add_user_query = "INSERT INTO users (firstname, lastname, email, password, dob, gender) ";
                            $add_user_query .= "VALUES ('$name', '$lastname', '$email', '$password', '$dob', '$gender' ) ";

                            $add_user_query_result = mysqli_query($connection, $add_user_query);

                                $_SESSION['name'] = $name;
                                $_SESSION['lastname'] = $lastname;
                                $_SESSION['email'] = $email;

                                header("Location: login.php");

                            }
                        }

                    ?>

                    <div class="form-group">
                        <label for="fistname">Name</label>
                        <input type="text" name="firstname" id="fistname" class="form-control">
                    </div>

                    <div class="form-group">
                        <label for="lastname">Lastname</label>
                        <input type="text" name="lastname" id="lastname" class="form-control">
                    </div>

                    <div class="form-group">
                        <label for="email">Email</label>
                        <input type="email" name="email" id="email" class="form-control">
                    </div>

                    <div class="form-group">
                        <label for="password">Password</label>
                        <input type="text" name="password" id="password" class="form-control">
                    </div>

                    <div class="form-group">
                        <label for="dob">Date of Birth</label>
                        <input type="date" name="dob" id="dob" class="form-control">
                    </div>

                    <div class="form-group">
                       <label for="gender">Male of Female</label>
                        <div class="radio">
                            <label><input type="radio" name="gender" value="male">Male</label>
                        </div>
                        <div class="radio">
                            <label><input type="radio" name="gender" value="female">Female</label>
                        </div>
                    </div>

                    <div class="form-group">
                        <input type="submit" name="register" class="btn btn-primary" value="Register">
                    </div>

                </form>
            </div>
        </div>
    </div>

</div>


<?php include "includes/footer.php"; ?>

Login Form Code:

<?php include "includes/header.php"; ?>
<?php include "includes/navigation.php"; ?>
<?php session_start(); ?>


<div class="container">
    <div class="row">
        <div class="col-xs-6 col-xs-offset-3">
            <div class="well">
                <h4>Login Form</h4>
                <form method="post" action="">

                   <?php

                        if(isset($_POST['login'])) {

                            $email = mysqli_real_escape_string($connection, $_POST['email']);
                            $password = mysqli_real_escape_string($connection, $_POST['password']);


                            $collect_users_info = "SELECT * FROM users WHERE email ='$email' ";

                            $collect_users_info_results = mysqli_query($connection, $collect_users_info);


                            $db_password = "";


                            while($row = mysqli_fetch_assoc($collect_users_info_results)) {
                                $db_name = mysqli_real_escape_string($connection, $row['name']);
                                $db_email = mysqli_real_escape_string($connection, $row['email']);
                                $db_password = mysqli_real_escape_string($connection, $row['password']);
                            }

                            if(password_verify($password, $db_password) && $email == $db_email) {

                                    header("Location: admin/index.php");

                            } else {

                                    echo "<h5>Email or Password is Incorect</h5>";

                                }

                        }

                    ?>

                    <div class="form-group">
                        <label for="email">Email</label>
                        <input type="email" name="email" id="email" class="form-control">
                    </div>

                    <div class="form-group">
                        <label for="password">Password</label>
                        <input type="text" name="password" id="password" class="form-control">
                    </div>


                    <div class="form-group">
                        <input type="submit" name="login" class="btn btn-primary" value="Login">
                    </div>

                </form>
            </div>
        </div>
    </div>

</div>


<?php include "includes/footer.php"; ?>

Admin Page:

<?php include "includes/admin_header.php"; ?>
<?php session_start(); ?>

<div id="wrapper">

<?php include "includes/admin_navigation.php"; ?>

        <div id="page-wrapper">

            <div class="container-fluid">

                <!-- Page Heading -->
                <div class="row">
                    <div class="col-lg-12">
                        <h1 class="page-header">

                            <?php echo "Welcome ". $_SESSION['name']; ?>
                        </h1>
                    </div>
                </div> 
            </div>
        </div>
    </div>

    <?php include "includes/admin_footer.php"; ?>

make sure you include the php file which contains the code for database connectivity is included in every other php page.