PHP - Fatal error: Call to a member function function() on a non-object in /ruta

<?php

include_once 'db_connect.php';

include_once 'psl-config.php';

$error_msg = "";

if (isset($_POST['username'], $_POST['email'], $_POST['p'])) {

    $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)) {

        $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) {

        $error_msg .= '<p class="error">Invalid password configuration.</p>';

    }

    $prep_stmt = "SELECT id FROM members WHERE email = ? LIMIT 1";

    $stmt = $mysqli->prepare($prep_stmt);

    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>';

        }

    } else {

        $error_msg .= '<p class="error">Database error</p>';

    }

    if (empty($error_msg)) {

        $random_salt = hash('sha512', uniqid(openssl_random_pseudo_bytes(16), TRUE));

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

        if ($insert_stmt = $mysqli->prepare("INSERT INTO members (username, email, password, salt) VALUES (?, ?, ?, ?)")) {

            $insert_stmt->bind_param('ssss', $username, $email, $password, $random_salt);

            if (! $insert_stmt->execute()) {

                header('Location: ../testing2/secure/phpSecure/error.php?err=Registration failure: INSERT');

                exit();

            }

        }

        header('Location: ../testing2/secure/phpSecure/register_success.php');

        exit();

    }

}