How can I make it possible that users with hashed passwords can login into my system?

login.php

<?php
include "db.php";
?>

<?php
session_start(); // Turns on the sessions.
?>

<?php
if (isset($_POST['login'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    $username = mysqli_real_escape_string($connection, $username);
    $password = mysqli_real_escape_string($connection, $password);

    $query = "SELECT * FROM users WHERE username = '{$username}' ";
    $select_user_query = mysqli_query($connection, $query);

    if (!$select_user_query) {
        die("Query failed" . mysqli_error($connection));
    }

    while($row = mysqli_fetch_assoc($select_user_query)) { //array.
        $db_user_id = $row['user_id'];
        $db_username = $row['username'];
        $db_user_password = $row['user_password'];
        $db_user_firstname = $row['user_firstname'];
        $db_user_lastname = $row['user_lastname'];
        $db_user_role = $row['user_role'];
    }

    if ($username === $db_username && $password === $db_user_password) {
        $_SESSION['username'] = $db_username; 
        $_SESSION['firstname'] = $db_user_firstname;
        $_SESSION['lastname'] = $db_user_lastname;
        $_SESSION['user_role'] = $db_user_role;

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

    } else {
        header("Location: ../index.php");
    }
}
?>

registration.php

<?php
include "includes/db.php";
include "includes/header.php"; 
?>

<?php
if (isset($_POST['submit'])) {

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

    // VALIDATION
    if (!empty($username) && !empty($email) && !empty($password)) {
        // Hashing
        $password = password_hash($password, PASSWORD_DEFAULT);

        $query = "INSERT INTO users (username, user_email, user_password, user_role) ";
        $query .= "VALUES('{$username}', '{$email}', '{$password}', 'subscriber' )";
        $register_user_query = mysqli_query($connection, $query);

        if (!$register_user_query) {
            die("QUERY FAILED " . mysqli_error($connection) . ' ' . mysqli_errno($connection));
        }

        $message = "Your registration has been submitted";

    } else {
        $message = "Fields cannot be empty";
    }

} else {
    $message = "";
}
?>


    <!-- Navigation -->
    <?php  
    include "includes/navigation.php"; 
    ?>


    <!-- Page Content -->
    <div class="container">

<section id="login">
    <div class="container">
        <div class="row">
            <div class="col-xs-6 col-xs-offset-3">
                <div class="form-wrap">
                <h1>Register</h1>
                    <form role="form" action="registration.php" method="post" id="login-form" autocomplete="off">
                        <h6 class="text-center"><?= $message; ?></h6>
                        <div class="form-group">
                            <label for="username" class="sr-only">Username</label>
                            <input type="text" name="username" id="username" class="form-control" placeholder="Enter Desired Username">
                        </div>
                         <div class="form-group">
                            <label for="email" class="sr-only">Email</label>
                            <input type="email" name="email" id="email" class="form-control" placeholder="somebody@example.com">
                        </div>
                         <div class="form-group">
                            <label for="password" class="sr-only">Password</label>
                            <input type="password" name="password" id="key" class="form-control" placeholder="Password">
                        </div>

                        <input type="submit" name="submit" id="btn-login" class="btn btn-custom btn-lg btn-block" value="Register">
                    </form>

                </div>
            </div> <!-- /.col-xs-12 -->
        </div> <!-- /.row -->
    </div> <!-- /.container -->
</section>


        <hr>



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