直接上代码MySqlDBSingle.class.php

<?php

/**
 * Created by PhpStorm.
 * User: Mengk
 * Date: 2018/8/29
 * Time: 10:38
 */
header("content-type:text/html;charset=utf-8");

class MySqlDBSingle
{
    private $host;                //主机IP
    private $port;                //端口号
    private $user;                //用户名
    private $pwd;                 //密码
    private $charset;             //字符编码
    private $dbname;              //连接的数据库
    private static $instance;    //保存MySQL的唯一实例

    private $link;                //保存数据库连接对象 可以省略

    /**
     * 初始化连接数据库参数
     * @param $config
     */
    private function initParams($config)
    {
        $this->host = isset($config['host']) ? $config['host'] : '';
        $this->port = isset($config['port']) ? $config['port'] : '';
        $this->user = isset($config['user']) ? $config['user'] : '';
        $this->pwd = isset($config['pwd']) ? $config['pwd'] : '';
        $this->charset = isset($config['charset']) ? $config['charset'] : 'utf8';
        $this->dbname = isset($config['dbname']) ? $config['dbname'] : '';

    }

    /**
     * 连接数据库
     */
    private function connect()
    {
        $this->link = @mysql_connect("{$this->host}:{$this->port}", $this->user, $this->pwd) or die('数据库连接失败');
    }

    /**
     * 此方法用来执行sql语句
     * 如果是数据查询语句 成功返回结果集 失败返回false
     * 如果是数据操作语句 成功返回true   失败返回false
     * @param $sql
     * @return resource
     */
    public function query($sql)
    {
        if (!$result = mysql_query($sql, $this->link)) {
            echo 'sql语句执行失败<br>';
            echo '错误编号：' . mysql_errno(), '<br>';
            echo '错误信息：' . mysql_error(), '<br>';
            echo '错误的sql语句：' . $sql, '<br>';
            exit;
        }
        return $result;
    }

    /**
     * 设置字符集编码
     */
    private function setCharset()
    {
        $sql = "set names {$this->charset}";
        $this->query($sql);
    }

    /**
     * 选择数据库
     */
    private function selectDB()
    {
        $sql = "use {$this->dbname}";
        $this->query($sql);
    }

    /**
     * 私有的构造方法防止类的外部实例化对象
     * MySqlDBSingle constructor.
     * @param $config   连接数据库的参数
     */
    private function __construct($config)
    {
        $this->initParams($config);
        $this->connect();
        $this->setCharset();
        $this->selectDB();
    }

    /**
     * 私有的克隆方法 用来阻止类的外部克隆对象
     */
    private function __clone()
    {
        // TODO: Implement __clone() method.
    }

    /**
     * 公有的静态方法获取唯一的实例
     * @param array $config
     * @return MySqlDBSingle
     */
    public static function getInstance($config = array())
    {
        if (!self::$instance instanceof MySqlDBSingle) {
            self::$instance = new MySqlDBSingle($config);
        }
        return self::$instance;
    }

    /**
     * 从数据库获取所有数据
     * @param $sql
     * @param string $fetch_type    assoc|row|array
     * @return array                将表中的数据匹配成二维数组返回
     */
    public function fetchAll($sql, $fetch_type = 'assoc')
    {
        $rs = $this->query($sql);
        $fetch_types = array('assoc','row','array');
        if (!in_array($fetch_type,$fetch_types)) {
            $fetch_type = 'assoc';
        }
        $fetch_fun = 'mysql_f etch_'.$fetch_type;
        $array = array();
        while ($rows = $fetch_fun($rs)) {
            $array[] = $rows;
        }
        return $array;
    }

    /**
     * 查询列表的第一条记录
     * @param $sql
     * @param string $fetch_type
     * @return mixed|null
     */
    public function fetchRow($sql, $fetch_type = 'assoc') {
        $rs = $this->fetchAll($sql, $fetch_type);
        if(!empty($rs)) {
            return $rs[0];
        }
        return null;
    }

    /**
     * 查询列表的第一行第一列
     * @param $sql
     * @return null
     */
    public function fetchColumn($sql) {
        $rs = $this->fetchRow($sql,'row');
        if(!empty($rs)) {
            return $rs[0];
        }
        return null;
    }

}

$config = array(
    'host' => 'localhost',
    'port' => '3306',
    'user' => 'root',
    'pwd' => 'root',
    'charset' => 'utf8',
    'dbname' => 'mydatabase'
);
$db = MySqlDBSingle::getInstance($config);

$rs = $db->fetchAll("select * from my_student",'assoc');
echo '<pre>';
var_dump($rs);echo '<hr>';

$rs = $db->fetchRow("select * from my_student");
echo '<pre>';
var_dump($rs);echo '<hr>';

$rs = $db->fetchColumn("select * from my_student");
echo '<pre>';
var_dump($rs);echo '<hr>';