<pre>
<?php
ini_set('display_errors',1);
error_reporting(E_ALL);

class Database
{
  protected $db_host = "host";
  protected $db_user = "user";
  protected $db_pass = "password";
  protected $db_name = "database";
  protected $connection = false;

  protected $mysqli;

  public function __construct()
  {
    if(!$this->connection)
    {
      $this->mysqli = new mysqli($this->db_host, $this->db_user, $this->db_pass, $this->db_name);
      if($this->mysqli->connect_errno)
      {
        echo "Не удалось подключится к MySQL: " . $mysqli->connect_error;
      }
      else
      {
        if(!$this->mysqli->set_charset("utf8"))
        {
          printf("Ошибка при загрузке набора символов utf8: %s\n", $this->mysqli->error);
          return false;
        }
        else
        {
          printf("Текущий набор символов: %s\n", $this->mysqli->character_set_name());
          $this->connection = true;
          return true;
        }
      }
    }
    else
    {
      return $this->mysqli;
      return true;
    }
  }

    public function tableExist($table)
  {
	$tablesInDatabase = $this->mysqli->query('SHOW TABLES FROM ' . $this->db_name . ' LIKE "' . $table . '"');
	if($tablesInDatabase)
	{
		if($tablesInDatabase->num_rows == 1)
		{
			echo "Существует таблица " . $table . " в базе данных";
			return true;
		}
		else
		{
			echo "Нет доступна к таблице " . $table;
			return false;
		}
	}
  }

  public function setDatabase($name)
  {
    if(!$this->connection)
    {
		$this->mysqli;
        $this->connection = false;
        $this->db_name = $name;
        $this->tableExist($name);
    }
  }

}

$db = new Database;
$db->setDatabase('calculator');

?>