How to connect in this PHP database tutorial?

Question

I'm following this tutorial, which includes:

include('db.php');

$db = new db();

In the tutorial does not show how new db() is created or the db.php file, however I do on my system already have config.local.php which contains the following:

$config['db_host'] = 'localhost';
$config['db_name'] = 'database_name';
$config['db_user'] = 'database_user';
$config['db_password'] = 'c@W)ukmd[0bm';
$config['database_backend'] = 'mysqli';

I'm really new to this. How can I use my current config file? I've already included "config.local.php" but need to of course use my config style and do the connection.

PHP (adjusted)

    <?php
//if we got something through $_POST
if (isset($_POST['search'])) {
    // here you would normally include some database connection
    include('config.local.php');
    $db = new db();

    // never trust what user wrote! We must ALWAYS sanitize user input
    $word = mysql_real_escape_string($_POST['postcode_q']);
    $word = htmlentities($word);
    // build your search query to the database
    $sql = "SELECT description FROM cscart_postcode_location_descriptions WHERE cscart_postcode_location_descriptions LIKE '%" . $word . "%' ORDER BY cscart_postcode_location_descriptions LIMIT 1";
    // get results
    $row = $db->select_list($sql);
    if(count($row)) {
        $end_result = '';
        foreach($row as $r) {
            $result         = $r['cscart_postcode_location_descriptions'];
            // we will use this to bold the search word in result
            $bold           = '<span class="found">' . $word . '</span>';    
            $end_result     .= '<li>' . str_ireplace($word, $bold, $result) . '</li>';            
        }
        echo $end_result;
    } else {
        echo '<li>No results found</li>';
    }
}
?>

MySQLi Attempt (needs review)

<?php
//if we got something through $_POST
if (isset($_POST['postcode_locator_search'])) {
    // here you would normally include some database connection
    include('config.local.php');

    //Open a new connection to the MySQL server
    $mysqli = new mysqli($config['db_host'],$config['db_user'],$config['db_password'],$config['db_name']);

    //Output any connection error
    if ($mysqli->connect_error) {
        die('Error : ('. $mysqli->connect_errno .') '. $mysqli->connect_error);
    }

    // never trust what user wrote! We must ALWAYS sanitize user input
    $postcode_q = mysql_real_escape_string($_POST['postcode_q']);
    $postcode_q = htmlentities($postcode_q);

    //chained PHP functions
    $sql = $mysqli->query("SELECT description FROM cscart_postcode_location_descriptions WHERE cscart_postcode_location_descriptions LIKE '%" . $postcode_q . "%' ORDER BY cscart_postcode_location_descriptions LIMIT 1"); 

    if(count($sql)) {
        $end_result = '';
        foreach($sql as $r) {
            $result         = $r['cscart_postcode_location_descriptions'];
            // we will use this to bold the search word in result
            $bold           = '<span class="found">' . $word . '</span>';    
            $end_result     .= '<li>' . str_ireplace($word, $bold, $result) . '</li>';            
        }
        echo $end_result;
    } else {
        echo '<li>No results found</li>';
    }

    $mysqli->close();

}
?>

jQuery I've done:

$(function() {

    $(".postcode_locator_form .ty-btn-go").click(function() {
        // getting the value that user typed
        var searchString    = $("#postcode_locator_search").val();
        // forming the queryString
        var data            = 'postcode_locator_search='+ searchString;

        // if searchString is not empty
        if(searchString) {
            // ajax call
            $.ajax({
                type: "POST",
                url: "search_postcode.php",
                data: data,
                beforeSend: function(html) { // this happens before actual call
                    $("#results").html(''); 
                    $(".searched-postcode").html(searchString);
                },
                success: function(html){ // this happens after we get results
                    $("#results").show();
                    $("#results").append(html);
                },
                error: function(html){
                    $("#results").show();
                    $("#results").append(html);
                }
            });    
        }
        return false;
    });
});