FILE 1:

class databaseConnection {
    private $host = 'localhost';
    private $database = 'xxxx';
    private $username = 'xxxx';
    private $password = 'xxxx';
    public $mysqli;

    public function __CONSTRUCT() {
        $this->mysqli = new mysqli($this->host,$this->username,$this->password,$this->database);
    }
}


FILE 2:
class stuff {

    public $mysqli;

    function __CONSTRUCT() {
        $this->mysqli = new databaseConnection;
    }

    function createTable() {
        $query = 'CREATE TABLE IF NOT EXISTS '.$this->tableName.'(';
        $query .= 'key VARCHAR(15),';
        $query .= 'code INT(3),';
        $query .= 'image VARCHAR(255),';
        $query .= 'PRIMARY KEY (key)';
        $query .= ');';
        $this->mysqli($query);
    }
}