PHP Header with mail() function error

Question

Having a small problem with headers when I insert some activation code.

I have a registration form which salts the passwords and it works a treat. I decided to add an MD5 checksum activation code to verify the user's email address. It inserts it into the database just fine but when it executes the php mail function I get an error Warning: Cannot modify header information

Just need some advice on how to approach this problem. when I comment out the mail function (near bottom of the script) it all works fine.

psl-config file

define("HOST", "localhost");     // The host you want to connect to.
define("USER", "sec_user");    // The database username. 
define("PASSWORD", "password");    // The database password. 
define("DATABASE", "database");    // The database name.

define("CAN_REGISTER", "any");
define("DEFAULT_ROLE", "member");

define("SECURE", FALSE);    // FOR DEVELOPMENT ONLY!!!!
define("WEBSITE_URL", "http://localhost");
define("EMAIL", "noreply@email.com");

The register file

include_once 'db_connect.php';
include_once 'psl-config.php';

$error_msg = "";

if (isset($_POST['username'], $_POST['email'], $_POST['p'])) {
    // Sanitize and validate the data passed in
    $username = filter_input(INPUT_POST, 'username', FILTER_SANITIZE_STRING);
    $email = filter_input(INPUT_POST, 'email', FILTER_SANITIZE_EMAIL);
    $email = filter_var($email, FILTER_VALIDATE_EMAIL);
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        // Not a valid email
        $error_msg .= '<p class="error">The email address you entered is not valid</p>';
    }

    $password = filter_input(INPUT_POST, 'p', FILTER_SANITIZE_STRING);
    if (strlen($password) != 128) {
        // The hashed pwd should be 128 characters long.
        // If it's not, something really odd has happened
        $error_msg .= '<p class="error">Invalid password configuration.</p>';
    }

    // Username validity and password validity have been checked client side.
    // This should should be adequate as nobody gains any advantage from
    // breaking these rules.
    //

    $prep_stmt = "SELECT id FROM members WHERE email = ? LIMIT 1";
    $stmt = $mysqli->prepare($prep_stmt);

    // check existing email
    if ($stmt) {
        $stmt->bind_param('s', $email);
        $stmt->execute();
        $stmt->store_result();

        if ($stmt->num_rows == 1) {
            // A user with this email address already exists
            $error_msg .= '<p class="error">A user with this email address already exists.</p>';
            $stmt->close();
        }
        $stmt->close();
    } else {
        $error_msg .= '<p class="error">Database error Line 39</p>';
        $stmt->close();
    }

    // check existing username
    $prep_stmt = "SELECT id FROM members WHERE username = ? LIMIT 1";
    $stmt = $mysqli->prepare($prep_stmt);

    if ($stmt) {
        $stmt->bind_param('s', $username);
        $stmt->execute();
        $stmt->store_result();

        if ($stmt->num_rows == 1) {
            // A user with this username already exists
            $error_msg .= '<p class="error">A user with this username already exists</p>';
            $stmt->close();
        }
        $stmt->close();
    } else {
        $error_msg .= '<p class="error">Database error line 55</p>';
        $stmt->close();
    }

    // TODO: 
    // We'll also have to account for the situation where the user doesn't have
    // rights to do registration, by checking what type of user is attempting to
    // perform the operation.

    if (empty($error_msg)) {
        // Create a random salt
        //$random_salt = hash('sha512', uniqid(openssl_random_pseudo_bytes(16), TRUE)); // Did not work
        $random_salt = hash('sha512', uniqid(mt_rand(1, mt_getrandmax()), true));

        // Create salted password 
        $password = hash('sha512', $password . $random_salt);

        // Create a unique  activation code:
        $activation = md5(uniqid(rand(), true));

        //layout activation email
        $message = " To activate your account, please click on this link:\n\n";
        $message .= WEBSITE_URL . '/activate.php?email=' . urlencode($Email) . "&key=$activation";
        // Send the email:
        mail($Email, 'Registration Confirmation', $message, 'From:'.EMAIL);


        // Insert the new user into the database
        if ($insert_stmt = $mysqli->prepare("INSERT INTO members (username, email, password, salt, activation) VALUES (?, ?, ?, ?, ?)")) {
            $insert_stmt->bind_param('sssss', $username, $email, $password, $random_salt, $activation);
            // Execute the prepared query.
            if (! $insert_stmt->execute()) {
                header('Location: ../error.php?err=Registration failure: INSERT');
            }
        }
        header('Location: ./register_success.php');
    }
}