having an error while creating automatic paper generator software on php

Error is

Fatal error: Uncaught mysqli_sql_exception: Unknown database ‘aqpg_db’ in C:xampphtdocsaqpgclassesDBConnection.php:18 Stack trace: #0 C:xampphtdocsaqpgclassesDBConnection.php(18): mysqli->__construct(‘localhost’, ‘root’, ”, ‘aqpg_db’) #1 C:xampphtdocsaqpgclassesSystemSettings.php(8): DBConnection->__construct() #2 C:xampphtdocsaqpgclassesSystemSettings.php(229): SystemSettings->__construct() #3 C:xampphtdocsaqpgconfig.php(9): require_once(‘C:xampphtdocs…’) #4 C:xampphtdocsaqpgindex.php(1): require_once(‘C:xampphtdocs…’) #5 {main} thrown in C:xampphtdocsaqpgclassesDBConnection.php on line 18)

code is :

<?php
if(!defined('DB_SERVER')){
    require_once("../initialize.php");
}
class DBConnection{

    private $host = DB_SERVER;
    private $username = DB_USERNAME;
    private $password = DB_PASSWORD;
    private $database = DB_NAME;
    
    public $conn;
    
    public function __construct(){

        if (!isset($this->conn)) {
            
            $this->conn = new mysqli($this->host, $this->username, $this->password, $this->database);
            
            if (!$this->conn) {
                echo 'Cannot connect to database server';
                exit;
            }            
        }    
        
    }
    public function __destruct(){
        $this->conn->close();
    }
}

?>