php单例模式的mysql数据访问类

<?php
class DbMySql
{
     private $db;
     private $dbName;
     static $instance;
 
     private function __construct($dbHost, $dbName, $dbUser, $dbPwd, $charset = 'utf8')
    {
         $this -> db = mysql_connect($dbHost, $dbUser, $dbPwd) or die('Could not connect: ' . mysql_error());
         $this -> select_db($dbName);
         $this -> query("SET NAMES $charset");
 
         }
 
     private function __clone(){
    }
 
     public static function get_instance($dbHost, $dbName, $dbUser, $dbPwd, $charset)
    {
         if(!(self :: $instance instanceof self))
            {
             self :: $instance = new self($dbHost, $dbName, $dbUser, $dbPwd, $charset);
             }
         return self :: $instance;
         }
 
    /**
     * 查询表
     * 返回一个一维数组
     */
     public function get_one($sql)
    {
         $query = $this -> query($sql);
         $rs = $this -> fetch_array($query);
         $this -> free_result($query);
         return $rs ;
         }
 
    /**
     * 查询表
     * 返回一个二维数组
     */
     public function select($sql, $keyfield = '')
    {
         $array = array();
         $result = $this -> query($sql);
         while($r = $this -> fetch_array($result))
        {
             if($keyfield)
            {
                 $key = $r[$keyfield];
                 $array[$key] = $r;
                 }
            else
                {
                 $array[] = $r;
                 }
             }
         $this -> free_result($result);
         return $array;
         }
 
    /**
     * 修改数据
     * 
     * @param tableName $ 操作的表
     * @param array $ 信息数组
     * @param where $ 条件
     */
     function update($tableName, $array, $where = '')
    {
         if($where)
        {
             $sql = '';
             foreach($array as $k => $v)
            {
                 $sql .= ", `$k`='$v'";
                 }
             $sql = substr($sql, 1);
             $sql = "UPDATE $tablename SET $sql WHERE $where";
             }
        else
            {
             $sql = "REPLACE INTO $tablename(`" . implode('`,`', array_keys($array)) . "`) VALUES('" . implode("','", $array) . "')";
             }
         return $this -> query($sql);
         }
 
    /**
     * 添加数据
     * 
     * @param tableName $ 操作的表
     * @param array $ 信息数组
     * return int 信息id
     */
     function insert($tableName, $array)
    {
         $this -> query("INSERT INTO $tableName(`" . implode('`,`', array_keys($array)) . "`) VALUES('" . implode("','", $array) . "')");
         return $this -> insert_id();
         }
 
     public function query($sql)
    {
         return mysql_query($sql, $this -> db);
         }
 
     public function fetch_array($query, $result_type = MYSQL_ASSOC)
    {
         return mysql_fetch_array($query, $result_type);
         }
 
     public function select_db($dbName)
    {
         mysql_select_db($dbName , $this -> db) or die('Could not select database');
         $this -> dbName = $dbName;
         }
 
     public function close()
    {
         return mysql_close($this -> db);
         }
 
     public function free_result(& $query)
    {
         return mysql_free_result($query);
         }
 
     public function insert_id()
    {
         return mysql_insert_id($this -> db);
         }
 
     public function fetch_row($query)
    {
         return mysql_fetch_row($query);
         }
 
     public function affected_rows()
    {
         return mysql_affected_rows($this -> db);
         }
 
     public function num_rows($query)
    {
         return mysql_num_rows($query);
         }
    }
?>