<?php
session_start();
$name=$_POST['name'];
$password=$_POST['password'];
try{
  $pdo=new PDO("mysql:host=HOST_NAME;dbname=chat_db",
    "USER_NAME",
    "PASSWORD");
    $sql="SELECT user,password from chat_user where user=? and password=?";
    $stmt=$pdo->prepare($sql);
    $stmt->execute(array($name,$password));
    $result=$stmt->fetch(PDO::FETCH_ASSOC);

    if($name==$result['user']&&$password==$result['password']){
      $_SESSION['name']=$name;
      header('Location:talk.php');
    }else if($name!=$result['user']||$password!=$result['password']){
      header('Location:register.php');
    }
}catch (PDOException $e){
    echo $e->getMessage();
    exit;
}
 ?>