使用类中的PDO连接数据库

Question

I have two files file1.php and database.php. I have a class 'Connection' in database.php file. I am calling this function as passing query to class 'bar'. But I am getting two errors:

1.Notice: Undefined variable: conn... on line 22
2.Fatal error: Cannot access empty property in.. on line 22

These are the codes I am using. Thank you.

file1.php

require_once('database.php');
$c = new Connection();
$c->bar("Select id, firstname, lastname from users");
$c->execute();

database.php 

class Connection{
    public $conn;

    public function __construct()
    {
        $servername = "localhost";
        $username = "root";
        $password = "";
        $dbname = "testing_pdo";
        try {
            $this->conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
            $this->conn->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
            $this->conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
        } catch (Exception $e) {
            echo 'ERROR: ' . $e->getMessage();
        }
    }
    public function bar($sql)
    {
        $this->conn->prepare($sql);
        return $this->conn->execute();
    }
}

Answer 1

I finally made it out. I made few changes in both files.

database.php

class Connection{
    private $servername = "localhost";
    private $username = "root";
    private $password = "";
    private $dbname = "testing_pdo";

    private $conn;
    private $error;
    private $stmt;


    public function __construct()
    {
        try {
            $this->conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
            $this->conn->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
            $this->conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
        } catch (Exception $e) {
            $this->error = $e->getMessage();
        }
    }
    public function bar($sql)
    {
        $this->stmt = $this->conn->prepare($sql);
    }

    public function resultset(){
        $this->execute();
        return $this->stmt->fetchAll(PDO::FETCH_ASSOC);
    }
}

file1.php

include ('database.php');
$c = new Connection();
$c->bar("Select id, firstname, lastname from users");
$c->resultset();