index.php

<?php
require "UserManager.php";

$userManager = new UserManager();

$userManager->insert("Sasha", "superPassword");

var_dump($userManager->selectAll());

User.php

<?php

class UserManager
{
  /**
   *@var \PDO 
   */
  private $pdo;
  
  public function __construct(){
    $this->pdo = new PDO('mysql:host=localhost;dbname=test', 'username', 'password');
  }
  
  public function insert($name, $password){
    $statement = $this->pdo->prepare("INSERT INTO `users` (`id`, `name`, `password`) VALUES (null, :name, :password)");
    $statement->bindValue("name", $name);
    $statement->bindValue("password", $password);
    $statement->execute();
  }
  
  public function selectAll(){
    $statement = $this->pdo->query("SELECT * FROM users");
    return $statement->fetchAll();
  }
  
}