PHP单例模式

<?php
class db {
    public $conn;
    public static $sql;
    public static $instance=null;
    private function __construct(){
        require_once('db.config.php');
        $this->conn = mysql_connect($db['host'],$db['user'],$db['password']);
        if(!mysql_select_db($db['database'],$this->conn)){
            echo "失败";
        };
        mysql_query('set names utf8',$this->conn);    
    }


    public static function getInstance(){
    if(is_null(self::$instance)){
        self::$instance = new self();
    }
        return self::$instance;
    }


    /**
    * 查询数据库
    */
    public function select($table,$condition=array(),$field = array()){
        $where='';
        if(!empty($condition)){

            foreach($condition as $k=>$v){
                $where.=$k."='".$v."' and ";
            }
            $where='where '.$where .'1=1';
        }

        $fieldstr = '';
        if(!empty($field)){
            foreach($field as $k=>$v){
                $fieldstr.= $v.',';
            }
            $fieldstr = rtrim($fieldstr,',');
        } else {
            $fieldstr = '*';
        }

        self::$sql = "select {$fieldstr} from {$table} {$where}";
        $result=mysql_query(self::$sql,$this->conn);
        $resuleRow = array();
        $i = 0;
        while($row=mysql_fetch_assoc($result)){
            foreach($row as $k=>$v){
                $resuleRow[$i][$k] = $v;
            }
        $i++;
        }
        return $resuleRow;
    }

    //添加一条记录
    public function insert($table,$data) {
        $values = '';
        $data = '';
        foreach ($data as $k=>$v) {
            $values .= $k.',';
            $datas .= "'$v'".',';
        }
        $values = rtrim($values,',');
        $datas = rtrim($datas,',');
        self::$sql = "INSERT INTO {$table} ({$values}) VALUES ({$datas})";
        if(mysql_query(self::$sql)) {
            return mysql_insert_id();
        } else {
            return false;
        }
    }

    //修改一条记录
    public function update($table,$data,$condition=array()){
        $where='';
        if(!empty($condition)) {

        foreach($condition as $k=>$v) {
            $where.=$k."='".$v."' and ";
        }
            $where='where '.$where .'1=1';
        }

        $updatastr = '';
        if(!empty($data)) {
        foreach($data as $k=>$v) {
            $updatastr.= $k."='".$v."',";
        }
            $updatastr = 'set '.rtrim($updatastr,',');
        }

        self::$sql = "update {$table} {$updatastr} {$where}";
        return mysql_query(self::$sql);
    }

    //删除记录
    public function delete($table,$condition) {
        $where='';
        if(!empty($condition)) {

            foreach($condition as $k=>$v) {
                $where.=$k."='".$v."' and ";
            }
            $where='where '.$where .'1=1';
        }
        self::$sql = "delete from {$table} {$where}";
        return mysql_query(self::$sql);
    }

    public static function getLastSql() {
        echo self::$sql;
    }
}

$db = db::getInstance();
//$list = $db->select('demo',array('name'=>'tom','password'=>'ds'),array('name','password'));
//echo $db->insert('demo',array('name'=>'最近你啦','password'=>'123'));
//echo $db->update('demo',array("name"=>'xxx',"password"=>'123'),array('id'=>1));
echo $db->delete('demo',array('id'=>'2'));
db::getLastSql();
echo "<pre>";
?>