PHP数据库连接错误，无法获取数据库类实例

I'm trying to perform CRUD operation by using instance of database class I've created here(class db), but I'm not getting database connection instance into another class, I'm using Netbeans IDE and MySql Database

<?php
class db
{
    private $dsn="mysql:host=localhost;dbname=db_stud";
    private $username="root";
    private $password='';  
    protected $connection;

    public function openConnection() 
    {
        try
        {
            $this->connection=new PDO($this->dsn, $this->username, $this->password);
            // set the PDO error mode to exception
            $this->connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);   

            return $this->connection;
        } catch (PDOException $ex) {
            print("Error ".$ex->getMessage()."<br/>");
            die();
        }
    }

    public function closeConnection()
    {
        $this->connection=null;
    }
}
?>

Given below is CRUD class

<?php
require_once("./db6.php");
class crud
{
    public $id;#function to insert record into database
    protected $dbcon;

    public function insert($studentArr)
    {
        $sql="INSERT INTO 'student' ('fname','lname','gender','batch','address','mobile'"
    . ",'age','dob','profilepic','fees') VALUES(:fname,:lname,:gender,:batch,:address"
    . ":mobile,:age,:dob,:profile,:fees)";
        $this->dbcon=new db;
        $db= $this->dbcon->openConnection();

        $result= $db->prepare($sql);
        $pdoresult=$result->execute(array(":fname"=>$fname,":lname"=>$lname,":gender"=>$gender,":batch"=>$std,":address"=>$address,
    ":mobile"=>$mobile,":age"=>$age,":dob"=>$dob,":profile"=>$profile,":fees"=>$fees));
        if($pdoresult)
        {
            $lastid=$db->lastInsertId();
            $this->id=$lastid;
        }
    }
}