If Username & Password Are Correct, Start Session

I am working on a basic login system that allows registration / sign in storing users in a MySQL database. I have it so people can register, it stores the username and a hashed password in the DB. When someone logs in, it shows a success message or an error message depending on if the username and password matched up.

My problem lies in the login function. I need to create the user session and redirect them to the logged-in only section of the site, instead of displaying the success message that is currently there. I'm unsure of how to do that...

Here is my code:

Login / Register Functions

function login($username, $password) {
    $userpass = sha1($password);
    $result = mysqli_query($con, "SELECT * FROM members WHERE username='$username' AND password='$userpass'");
    while($row = mysqli_fetch_array($result)) {
        $success = true;
    }
    if($success == true) {
        echo 'Success!';
    } else {
        echo '<div class="alert alert-danger">Oops! It looks like your username and/or password are incorrect. Please try again.</div>';
    }
} // END LOGIN FUNCTION

function register($username, $password) {
    $userpass = sha1($password);

    // Check if Username Exists
    $result = mysqli_query($con,"SELECT * FROM members WHERE username='$username'");
    while($row = mysqli_fetch_array($result)) {
        $userexist = 1;
    }
    if($userexist > 0) {
        echo '<div class="alert alert-danger">Sorry, it looks like that username is already taken.</div>';
    } else {
        $newmember = "INSERT INTO members SET username='$username', password='$userpass'";
        if(mysqli_query($con,$newmember)) {
            echo '<div class="alert alert-success">Congrats! You can now log in using your username and password</div>';
        }
    }
}

think like this

function login($username, $password) {
    $userpass = sha1($password);
    $result = mysqli_query($con, "SELECT * FROM members WHERE username='$username' AND password='$userpass'");
    while($row = mysqli_fetch_array($result)) {
        $success = true;
    }
    if($success == true) {
        $_SESSION['username']= $username; 
        //redirect to home page
    } else {
        echo '<div class="alert alert-danger">Oops! It looks like your username and/or password are incorrect. Please try again.</div>';
    }
} // END LOGIN FUNCTION