Обертка для работы с базой

<?php

class DB extends PDO {
    private $_host = '127.0.0.1',
        $_database = 'vk',
        $_username = 'root',
        $_password = '',
        $_tablePrefix = '';

    private function __construct() {
        $dsn = "mysql:dbname={$this->_database};host=" . $this->_host;
        parent::__construct($dsn, $this->_username, $this->_password);
        $this->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
     }

     static function getInstance() {
        static $db;
        if ($db == null) {
            $db = new self;
        }
        return $db;
     }

     function addTablePrefix($table_name) {
        return '`' . $this->_tablePrefix . $table_name . '`';
     }

     function insert($table_name, $data) {
        $sql = 'INSERT INTO ' . $this->addTablePrefix($table_name);
        $val = array_values($data);
        // если массив ассоциативный
        if ($val != $data) {
            $sql .= ' (`' . implode('`,`', array_keys($data)) . '`)';
        }
        $sql .= ' VALUES(' . implode(',', array_fill(0, count($val), '?')) . ');';
        $stmt = $this->prepare($sql);
        $stmt->execute($val);
        return $this->lastInsertId();
    }

    function update($table_name, $data, $id, $id_name = 'id') {
        $id = (int) $id;
        $items = array();
        foreach (array_keys($data) as $v) {
            $items[] = "`$v`=?";
        }
        $items = implode(',', $items);
        $sql = "UPDATE {$this->addTablePrefix($table_name)} SET $items WHERE `$id_name`=$id;";
        $stmt = $this->prepare($sql);
        $stmt->execute(array_values($data));
        return $stmt->rowCount();
    }

    function delete($table_name, $id, $id_name = 'id') {
        $id = (int) $id;
        $sql = "DELETE FROM {$this->addTablePrefix($table_name)} WHERE `$id_name`=$id;";
        $stmt = $this->query($sql);
        return $stmt->rowCount();
    }
}