Example PHP Singleton DB Class

class Db {
    // The class instance
    protected static $instance;

    // A connection to the db
    private $connection;

    // Function to create and return an instance of the Db class
    public static function getInstance(){
        if( !(self::$instance) ){
                    self::$instance = new self();
                }
                return self::$instance;
    }

    // Private class constructor prevents this class being instantiated other than by the getInstance() call
    private function __construct(){
        // Let's connect to the database here
                $config = parse_ini_file('./config.ini');
            $this->connection = new mysqli('localhost',$config['username'],$config['password'],$config['dbname']);

        // If connection was not successful, handle the error
            if($this->connection === false) {
                    // Handle error - notify administrator, log to a file, show an error screen, etc.
                    throw new Exception('Database connection failed');
            }
    }

    /**
        * Query the database
        *
        * @param $query The query string
        * @return mixed The result of the mysqli::query() function
        */
        public function query($query) {
            // Query the database
            $result = $this->connection->query($query);
            return $result;
        }

        /**
        * Fetch rows from the database (SELECT query)
        *
        * @param $query The query string
        * @return bool False on failure / array Database rows on success
        */
        public function select($query) {
            $rows = array();
            $result = $this->query($query);
            if($result === false) {
                    return false;
            }
            while ($row = $result -> fetch_assoc()) {
                    $rows[] = $row;
            }
            return $rows;
        }

        /**
        * Fetch the last error from the database
        *
        * @return string Database error message
        */
        public function error() {
            return $this->connection->error;
        }

        /**
        * Quote and escape value for use in a database query
        *
        * @param string $value The value to be quoted and escaped
        * @return string The quoted and escaped string
        */
        public function quote($value) {
            return "'" . $this->connection->real_escape_string($value) . "'";
        }

    // This prevents a bug in PHP to stop this class being cloned
    private function __clone() {}
}



try{
    // Our database object
    $db = Db::getInstance();

    // Quote and escape form submitted values
    $name = $db->quote($_POST['username']);
    $email = $db->quote($_POST['email']);

    // Insert the values into the database
    $result = $db->query("INSERT INTO `users` (`name`,`email`) VALUES (" . $name . "," . $email . ")");
}
catch (Exception $e){
    // Handle error - notify administrator, log to a file, show an error screen, etc.
    error_log($e->getMessage());
}