PHP单例模式封装 MySQLI

<?php
class MySQLDB {
    private $host;                // 主机地址
    private $port;                // 端口号
    private $user;                // 用户名
    private $pwd;                // 密码
    private $dbname;            // 数据库名
    private $charset;            // 字符集
    private $link;                // 连接对象
    private static $instance;
    private function __construct($param){
        $this->initParam($param);
        $this->initConnect();
    }
    private function __clone(){

    }
    // 获取单例
    public static function getInstance($param= array()){
        if(!self::$instance instanceof self){
            self::$instance= new self($param);
        }
        return self::$instance;
    }
    // 初始化参数
    private function initParam($param){
        $this->host= $param['host']??'127.0.0.1';
        $this->port= $param['port']??'3306';
        $this->user= $param['user']??'';
        $this->pwd= $param['pwd']??'';
        $this->dbname= $param['dbname']??'';
        $this->charset= $param['charset']??'utf8';
    }
    // 连接数据库
    private function initConnect(){
        $this->link= @mysqli_connect($this->host, $this->user, $this->pwd, $this->dbname, $this->port);
        if(mysqli_connect_error()){
            echo '数据库连接失败<br>';
            echo '错误信息：'.mysqli_connect_error(),'<br>';
            echo '错误码：'.mysqli_connect_errno(),'<br>';
            exit;
        }else{
            echo '数据库连接成功！';
        }
        mysqli_set_charset($this->link, $this->charset);    
    }
    // 数据库的增删改查
    private function execute($sql){
        if(!$rs= mysqli_query($this->link, $sql)){
            echo 'SQL语句执行失败<br>';
            echo '错误信息：'.mysqli_error($this->link),'<br>';
            echo '错误码：'.mysqli_errno($this->link),'<br>';
            echo '错误的SQL语句：'.$sql,'<br>';
            exit;
        }
        return $rs;
    }
    // 执行增删改语句
    public function exec($sql) {
        $key=substr($sql,0,6);
        if(in_array($key,array('insert','update','delete')))
            return $this->execute($sql);
        else{
            echo '非法访问<br>';
            exit;
        }
    }
    //获取自动增长的编号
    public function getLastInsertId() {
        return mysqli_insert_id($this->link);
    }
    //执行查询语句
    private function query($sql) {
        if(substr($sql,0,6)=='select' || substr($sql,0,4)=='show' || substr($sql,0,4)=='desc'){
            return $this->execute($sql);
        }else{
            echo '非法访问<br>';
            exit;
        }
    }
    //匹配所有数据
    public function fetchAll($sql,$type='assoc') {
        $rs=$this->query($sql);
        $type=$this->getType($type);
        return mysqli_fetch_all($rs,$type);
    }
    //匹配一维数组
    public function fetchRow($sql,$type='assoc') {
        $list=$this->fetchAll($sql,$type);
        if(!empty($list))
            return $list[0];
        return array();
    }
    //匹配一行一列
    public function fetchColumn($sql) {
        $list=$this->fetchRow($sql,'num');
        if(!empty($list))
            return $list[0];
        return null;
    }
    //获取匹配类型
    private function getType($type) {
        switch($type){
            case 'num':
                return  MYSQLI_NUM;
            case 'both':
                return  MYSQLI_BOTH;
            default:
                return  MYSQLI_ASSOC;
        }
    }
}

$param= array(
    'user' => 'root',
    'pwd' => '',
    'dbname' => 'sel'
);
$db= MySQLDB::getInstance($param);
// 查询所有信息
$listAll= $db->fetchAll('select * from news');
var_dump($listAll);
// 查询一条记录
$listRow= $db->fetchRow('select * from news');
var_dump($listRow);
// 查询一个字段
$listColumn= $db->fetchColumn('select * from news');
var_dump($listColumn);
?>