How to connect mysql database using PHP oop way?

I am currently learning PHP and trying to figure out how to connect my MySQL database named (lead_gen_business) using OOP. I was given a task to get the specific output by the way. My problem is I am having a hard time connecting to my database. For the context of the code. I created a database and the code inside is a pre-built function that I modified for OOP.

Database

class Database
{
    public $connection;

    public function __construct()
    {

        if (!defined('DB_HOST')) {
            define('DB_HOST', 'localhost');
        }

        if (!defined('DB_USER')) {
            define('DB_USER', 'root');
        }

        if (!defined('DB_PASS')) {
            define('DB_PASS', 'Yourpass');
        }
    }

    public function connect($db_name)
    {
        $this->connection = new mysqli(DB_HOST, DB_USER, DB_PASS, $db_name);
    }

    /*-----------------------END OF CONNECTION PROCESS------------------------*/
    /*----------------------DATABASE QUERYING FUNCTIONS-----------------------*/
    //SELECT - used when expecting single OR multiple results
    //returns an array that contains one or more associative arrays
    function fetch_all($query)
    {
        $data = array();
        global $connection;
        $result = $connection->query($query);
        while ($row = mysqli_fetch_assoc($result)) {
            $data[] = $row;
        }
        return $data;
    }
    //SELECT - used when expecting a single result
    //returns an associative array
    function fetch_record($query)
    {
        global $connection;
        $result = $connection->query($query);
        return mysqli_fetch_assoc($result);
    }
    //used to run INSERT/DELETE/UPDATE, queries that don't return a value
    //returns a value, the id of the most recently inserted record in your database
    function run_mysql_query($query)
    {
        global $connection;
        $result = $connection->query($query);
        return $connection->insert_id;
    }
    //returns an escaped string. EG, the string "That's crazy!" will be returned as "That's crazy!"
    //also helps secure your database against SQL injection
    function escape_this_string($string)
    {
        global $connection;
        return $connection->real_escape_string($string);
    }
}

    require_once('database.php');
    
    class Sites extends Database
    {
    
        public $client;
        public $havingArr = [];
        public $count = "COUNT(*)";
        public $id;
    
        public function select($myClient)
        {
            // Assuming $myClient is an array containing column names
            foreach ($myClient as $data) {
                $this->client = $data;
            }
            return $this;
        }
    
        public function group_by($siteId)
        {
            $this->id = $siteId;
            return $this;
        }
    
        public function having($paraCount, $greaterThan, $number)
        {
            // Building having condition
            $this->havingArr[] = "$paraCount $greaterThan $number";
            return $this;
        }
    
    
        public function get()
        {
            // $havingCondition = implode(" AND ", $this->havingArr);
            // $query = "SELECT {$this->client}, {$this->count} FROM sites
            //  GROUP BY {$this->id} HAVING $havingCondition";
            $query = "SELECT * FROM sites";
            // Assuming $this->connection->fetch_all is a method that executes the query and fetches all rows
            $result = $this->fetch_all($query);
    
            return $result;
        }
    }

And lastly…

    include('database.php');
    include('sites.php');
    
    
    class Query_builder extends Sites
    {
    
        public function __construct($db_name)
        {
    
            $this->connect($db_name);
        }
    
        public function show_get()
        {
            $datas = $this->get();
    
            foreach ($datas as $data) {
                echo $data['site_id'] . "<br>";
                echo $data['domain_name'] . "<br>";
            }
    
            return $this;
        }
    }
    
    $show = new Query_builder('lead_gen_business');
    
    var_dump($show->show_get());

I got a lot of errors before this and the last error I got is this.

Fatal error: Uncaught Error: Call to a member function query() on null in C:wamp64wwwOOP FundamentalsAdvance OOPQuery Builderdatabase.php on line 38

Sorry if it’s too long. I already spent a day trying to figure out this independently. Any answer is much appreciated! Thanks in advance.