<?php
    define("DB_HOST", "127.0.0.1");
    define("DB_USER", "root");
    define("DB_PASS", "");
    define("DB_NAME", "sample_db");

    $db = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);

    echo (!$db->connect_error) ? NULL : die("<pre>Unable to connect to the MySQL Server -> $db->connect_error</pre>");
?>

<?php
    public $db;

    class MySQLqueries {
        public function samplefunction($queryString) {
            $sqlQry = mysqli->query($queryString);

            return ($sqlQry) ? "<pre>Query Executed Successfully</pre>" : die("<pre>An error occured -> $db->error</pre>");
        }
    }
?>

<?php
    error_reporting(0);
    date_default_timezone_set("Asia/Manila");

    require 'connection.db.php';
    require 'sampleclass.class.php';
?>

<?php
    require 'includes.inc.php';

    $todo = new MySQLqueries;
    echo $todo->samplefunction("SELECT `sample_column` FROM `sample_table` WHERE `sample_column` = 'sample_value';");
?>

Class DB {

    function __construct($host, $user, $pass, $db) {
        return $this->connect($host, $user, $pass, $db);
    }

    function connect($host, $user, $pass, $db) {
        //..connect and all.
    }

    //...the rest of your functions/class...
}

Class Foo {

    $private $db;

    // your construct method here will ONLY except a `DB` class instance/object as $db.
    // Try it with anything else and learn from the errors to understand what I mean.
    function __construct(DB $db){
        $this->db = $db;
    }

}

$db = new DB($host, $user, $pass, $db);
// you can error check it here

$foo = new Foo($db);// inject the $db object.

include('connection.db.php');

class MySQLqueries {
        public function samplefunction($queryString) {
            global $db;
            $sqlQry = mysqli->query($queryString);

            return ($sqlQry) ? "<pre>Query Executed Successfully</pre>" : die("<pre>An error occured -> $db->error</pre>");
        }
}

class MysqlConn
{
    public static $db;
}

MysqlConn::$db = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);

<?php
class MysqlConn
{
    protected
        static $DB_HOST = "127.0.0.1",
        static $DB_USER = "root",
        static $DB_PASS = "",
        static $DB_NAME = "sample_db";

    protected static $db;

    public static function getDB()
    {
        if (!isset(self::$db) {
            self::$db = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);

            if ($db->connect_error) {
                die("<pre>Unable to connect to the MySQL Server -> $db->connect_error</pre>");
            }
        }
        return self::$db;
    }
}

MysqlConn::getDB();

$db = MysqlConn::getDB();

require_once 'connection.db.php';

        class MySQLqueries {
                public function samplefunction($queryString) {
                    global $db;
                    //You can now use $db variable as you want
                    print_r($db);
                    $sqlQry = mysqli->query($queryString);

                    return ($sqlQry) ? "<pre>Query Executed Successfully</pre>" : die("<pre>An error occured -> $db->error</pre>");
                }
        }

<?php
class connection
{

    public $servername = "localhost";
    public $username = "root";
    public $password = "root";
    public $dbname = "test";
    function __construct($servername, $username, $password, $dbname)
        {
            return $this->connect($servername, $username, $password, $dbname);
        }

    function connect($servername, $username, $password, $dbname)
        {
            $conn = new mysqli($servername, $username, $password, $dbname);
        }

class cc extends connection
{
private $conn;

function __construct(connection $conn)
        {
            $this->conn = $conn;
        }

}
}
$conn = new connection($servername, $username, $password, $dbname);
$obj = new connection();
echo $obj->connect();
$ccc = new cc($conn);// inject the $conn object.

?>