MySQL class implemented by PHP that supports reading and writing separation

/**
* MySQL read-write separation class
* $db_config = array(
* 'master' => array('host'=>'localhost:3306','user'=>'admin','passwd'= >'123456','db'=>'stat'),
* 'slave' => array(
* array('host'=>'localhost:3307','user'=>' admin','passwd'=>'123456','db'=>'stat'),
* array('host'=>'localhost:3308','user'=>'admin', 'passwd'=>'123456','db'=>'stat')
* )
* );
*
* Note: If there are multiple slaves, randomly connect one of them
* Last edit: bbs .it-home.org
*/
/*
$db_config = array(
    'master' => array('host'=>'localhost:3306','user'=>'admin','passwd'=>'123456','db'=>'stat'),
    'slave'  => array(
                    array('host'=>'localhost:3307','user'=>'admin','passwd'=>'123456','db'=>'stat'),
                    array('host'=>'localhost:3308','user'=>'admin','passwd'=>'123456','db'=>'stat')
                )
);
$db = MySQL::getInstance('','r-w');
$sql = "select * from admin";
$rs = $db->query($sql);
while ($row = $db->fetch($rs)){
    echo "uid:".$row['uid']."   ".$row['userName']."";
}
echo "";
*/
class MySQL
{
    private static $_instance = null;//数据库连接实例
    private static $_master = null;//主数据库连接实例
    private static $_slave = null;//重数据库连接实例

    public $_config = array();//数据库连接配置信息
    public $_res = null;//查询实例句柄
    public $_flag = '';//标识当前语句是在主还是重数据库上执行
    public $_link = null;

    /**
 * Single instance
 * Enter description here ...
 * @param unknown_type $dbname
 * @param unknown_type $mode
*/
    public static function & getInstance($dbname='',$mode='rw'){
        if (is_null(self::$_instance)){
            self::$_instance = new self();
            self::$_instance->__getConf();
            self::$_instance->connect($dbname,$mode);
        }

        return self::$_instance;
    }
    /**
 * Get database configuration information
 * Enter description here...
*/
    public function __getConf(){
        global $db_config;
        $this->_config['master'] = $db_config['master'];
        $this->_config['slave'] = $db_config['slave'];
    }

    /**
 * Database connection
 * Enter description here...
 * @param $dbname specifies the database name to connect to. By default, the library of the configuration file is connected
 * @param $mode rw means connecting to the main library, r-w means reading and writing separation
*/
    public function connect($dbname='',$mode = 'rw'){
        if($mode == 'rw'){
            if(is_null(self::$_master)){
                $this->_master = $this->_slave = $this->conn_master($dbname);
            }
        }else{
            if(is_null(self::$_master)){
                $this->_master = $this->conn_master($dbname);
            }
            if(is_null(self::$_slave)){
                $this->_slave = $this->conn_slave($dbname);
            }
        }
    }

    /**
 * Connect to the main database server
 * Enter description here...
*/
    public function conn_master($dbname=''){
        $_link = mysql_connect($this->_config['master']['host'],$this->_config['master']['user'],$this->_config['master']['passwd'],true) or die ("Connect ".$this->_config['master']['host']." fail.");
        mysql_select_db(empty($dbname)?$this->_config['master']['db']:$dbname,$_link) or die(" The DB name ".$this->_config['master']['db']." is not exists.");
        mysql_query("set names utf8",$_link);
        return $_link;
    }

    /**
 * Connect to slave database server
 * Enter description here...
*/
    public function conn_slave($dbname=''){
        $offset = rand(0,count($this->_config['slave'])-1);
        $_link = @mysql_connect($this->_config['slave'][$offset]['host'],$this->_config['slave'][$offset]['user'],$this->_config['slave'][$offset]['passwd'],true) or die(" Connect ".$this->_config['slave'][$offset]['host']." fail.");
        mysql_select_db(empty($dbname)?$this->_config['slave'][$offset]['db']:$dbname,$_link) or die(" The DB name ".$this->_config['slave'][$offset]['db']." is not exists.");
        mysql_query("set names utf8",$_link);
        return $_link;
    }
    /**
 * Execute database query
 * Enter description here ...
 * @param string $sql
*/
    public function query($sql,$master=true){
        if($master == true || (substr(strtolower($sql),0,6) != 'select') && $master == false){
            $this->_res = mysql_query($sql,$this->_master);
            if(!$this->_res){
                $this->_error[] = mysql_error($this->_master);
            }
            $this->_flag = 'master';
            $this->_link = $this->_master;
        } else {
            $this->_res = mysql_query($sql,$this->_slave);
            if(!$this->_res){
                $this->_error[] = mysql_error($this->_slave);
            }
            $this->_flag = 'slave';
            $this->_link = $this->_slave;
        }
        return $this->_res;
    }

    /**
 * Get a single row of records
 * Enter description here ...
 * @param mixed $rs
*/
    public function get($rs=''){
        if(empty($rs)){
            $rs = $this->_res;
        }
        return mysql_fetch_row($rs);
    }

    /**
 * Get multi-line records
 * Enter description here ...
 * @param mixed $rs
 * @param $result_type
*/
    public function fetch($rs = ''){
        if(empty($rs)){
            $rs = $this->_res;
        }
        return mysql_fetch_array($rs,MYSQL_ASSOC);
    }

    /**
 * Insert data
 * Enter description here ...
 * @param unknown_type $sql
*/
    public function add($sql){
        $rs = $this->query($sql);
        if($rs)
            return mysql_insert_id($this->_link);
        return false;
    }

    /**
 * Update data
 * Enter description here ...
 * @param unknown_type $sql
*/
    public function update($sql){
        if(empty($sql)) return false;
        $rs = $this->query($sql);
        if($rs)
            return $this->fetchNum();
        return false;
    }
    /**
 * Get the number of rows affected by the previous statement
 * Enter description here...
*/
    public function fetchNum(){
        return mysql_affected_rows($this->_link);
    }

    /**
 * Destructor, releases database connection resources
 * Enter description here...
*/
    public function __destruct(){
        mysql_close($this->_link);
    }
}