<?php
require_once('database.php');
$obj = new Database;
$obj->getConnection();
class Member{

private $username;
private $userpassword;

public function regist(){

if(isset($_POST['action'])&&$_POST['action']=='insert'){
try{

    $sql = "INSERT INTO userData(username,password)VALUES(:username,:password)";
    $stmh = $obj->prepare($sql);//the error occour here
    $stmh -> bindValue('username',$_POST['username'],PDO::PARAM_STR);
    $stmh -> bindValue('password',password_hash($_POST['userpassword'],PASSWORD_DEFAULT),PDO::PARAM_STR);
    $stmh -> execute();

    echo "登録しました<br>";
        }catch(PDOExcetion $e){
            echo "error ".$e->getMessage();
        }
    }

}

public function login(){
    $sql = "SELECT * FROM userData WHERE username=:usrname";
    $stmh =$obj->prepare($sql);//the error occour here
    $stmh ->bindValue(':usrname',$_POST['username'],PDO::PARAM_STR);
    $stmh->execute();
    $row = $stmh->fetch(PDO::FETCH_ASSOC);
    if($row['username'] == false){
        echo"login fail";
        exit;
    }
    $hash = $row['password'];
    if (false === password_verify($_POST['userpassword'],$hash)){
        echo"login fail";
        exit;
    }
   }
}

<?php
class Database{

protected $pdo;

public function __construct()
{
    $this->db_connect();
}

private function db_connect()
{
    $servername = "localhost";
    $username = "abc";
    $password = "abc";
    $dbname = "mydb";
    try
    {
      $this->pdo = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
      $this->pdo->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
      $this->pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES,false);
    }catch(PDOException $e)
        {
        die("error connection ".$e->getMessage());
        }
}
public function getConnection()
{
    return $this->pdo;
}
  }
  ?>