PHP 简单的数据库连接类(mysqli)

不能拿来直接用!!!
按需要做修改!

<?php
/**
*
**/
class Db
{
	private $lin;//数据库连接

	function __construct ()
	{
		$this->connect();
	}
	
	//这里需要做修改 DB_HOST DB_USER DB_PWD DB_DBNAME DB_CHARSET
	private function connect()
	{
		$this->lin = mysqli_connect(DB_HOST,DB_USER,DB_PWD) 
					 or die("数据库连接失败Error:".mysqli_error());
	  	mysqli_select_db($this->lin,DB_DBNAME) or die("指定数据库打开失败");
		mysqli_set_charset($this->lin,DB_CHARSET);
	}

	public function query($sql)
	{
		return mysqli_query($this->lin ,$sql);
	}

	//一条记录
	public function fetchOne($sql,$type=MYSQLI_ASSOC)
	{
		$result = $this->query($sql);
		return mysqli_fetch_array($result,$type);
	}

	//所有结果
	public function fetchAll($sql,$type=MYSQLI_ASSOC)
	{
		$result=$this->query($sql);
		while(@$row=mysqli_fetch_array($result,$type)){
			$rows[]=$row;
		}
		return $rows;
	}

	//新增
	public function add ($table ,$arr)
	{
		if ( is_array($arr) )
		{
			$columns = '';
			$values = '';
			foreach ( $arr as $k => $v )
			{
				$columns .= $k.',';
				$values .= "'".$v."'".',';
			}
			$columns = trim($columns ,',');
			$values = trim($values ,',');
			 
			$sql= "INSERT INTO $table ($columns) VALUES ($values)";
			$this->query($sql);
			return mysqli_insert_id($this->lin);
		}
	}

	//更新
	public function edit ($table ,$arr ,$where)
	{
		if ( is_array($arr) )
		{
			$modify = '';
			foreach ($arr as $key => $val) {
				$modify .= "`".$key."`='".$val."',";
			}
			$modify = trim($modify ,',');
			$sql = "UPDATE $table SET $modify WHERE $where";

			if ( $this->query($sql) )
			{
				return mysqli_affected_rows($this->lin);
			}
			return false;
		}
	}

	//删除记录
	public function del($table ,$where='')
	{
		$sql = "delete from {$table}";
		$sql .= $where===''? '': ' where '.$where;
		$this->query($sql);
		return mysqli_affected_rows($this->lin);
	}
	
	function __destruct ()
	{
		mysqli_close($this->lin);
	}
}