用单例模式来设计一个PHP数据库类

<?php
class nmdb 
{
private $link;
static private $_instance;
// 连接数据库
private function __construct($host, $username, $password)
{
$this->link = mysql_connect($host, $username, $password);
$this->query("SET NAMES 'utf8'", $this->link);
//echo mysql_errno($this->link) . ": " . mysql_error($link). "n";
//var_dump($this->link);
return $this->link;
}

private function __clone(){}

public static function get_class_nmdb($host, $username, $password)
{
        //$connector = new nmdb($host, $username, $password);
        //return $connector;

if( FALSE == (self::$_instance instanceof self) )
{
self::$_instance = new self($host, $username, $password);
}
return self::$_instance;
    }

// 连接数据表
public function select_db($database)
{
$this->result = mysql_select_db($database);
return $this->result;
}

// 执行SQL语句
public function query($query)
{
return $this->result = mysql_query($query, $this->link);
}

// 将结果集保存为数组
public function fetch_array($fetch_array)
{
return $this->result = mysql_fetch_array($fetch_array, MYSQL_ASSOC);
}

// 获得记录数目
public function num_rows($query)
{
return $this->result = mysql_num_rows($query);
}

// 关闭数据库连接
public function close()
{
return $this->result = mysql_close($this->link);
}

}
?>

这个类的使用如下：

$connector = nmdb::get_class_nmdb($host, $username, $password);
$connector -> select_db($database);

下面的类也可以参考下：

<?php
/*
* mysql 单例
*/
class mysql{
    private $host    ='localhost'; //数据库主机
    private $user     = 'root'; //数据库用户名
    private $pwd     = ''; //数据库用户名密码
    private $database = 'imoro_imoro'; //数据库名
    private $charset = 'utf8'; //数据库编码，GBK,UTF8,gb2312
    private $link;             //数据库连接标识;
    private $rows;             //查询获取的多行数组
    static $_instance; //存储对象
    /**
     * 构造函数
     * 私有
     */
    private function __construct($pconnect = false) {
        if (!$pconnect) {
            $this->link = @ mysql_connect($this->host, $this->user, $this->pwd) or $this->err();
        } else {
            $this->link = @ mysql_pconnect($this->host, $this->user, $this->pwd) or $this->err();
        }
        mysql_select_db($this->database) or $this->err();
        $this->query("SET NAMES '{$this->charset}'", $this->link);
        return $this->link;
    }
    /**
     * 防止被克隆
     *
     */
    private function __clone(){}
    public static function getInstance($pconnect = false){
        if(FALSE == (self::$_instance instanceof self)){
            self::$_instance = new self($pconnect);
        }
        return self::$_instance;
    }
    /**
     * 查询
     */
    public function query($sql, $link = '') {
        $this->result = mysql_query($sql, $this->link) or $this->err($sql);
        return $this->result;
    }
    /**
     * 单行记录
     */
    public function getRow($sql, $type = MYSQL_ASSOC) {
        $result = $this->query($sql);
        return @ mysql_fetch_array($result, $type);
    }
    /**
     * 多行记录
     */
    public function getRows($sql, $type = MYSQL_ASSOC) {
        $result = $this->query($sql);
        while ($row = @ mysql_fetch_array($result, $type)) {
            $this->rows[] = $row;
        }
        return $this->rows;
    }
    /**
     * 错误信息输出
     */
    protected function err($sql = null) {
        //这里输出错误信息
        echo 'error';
        exit();
    }
}
//用例
$db = mysql::getInstance();
$db2 = mysql::getInstance();
$data = $db->getRows('select * from blog');
//print_r($data);
//判断两个对象是否相等
if($db === $db2){
    echo 'true';
}
?>