<?php
session_start();
if( !empty($_POST['email']) && isset($_POST['email']) && !empty($_POST['senha']) && isset($_POST['senha']) ){
        $email = addslashes($_POST['email']);
        $senha = md5($_POST['senha']);

        try{
            $dsn = 'mysql:dbname=blog;host=localhost';
            $dbuser = 'root';
            $dbpass = 'root';

            $pdo = new PDO($dsn,$dbuser,$dbpass);
            $sql = $pdo->query("SELECT * FROM usuarios WHERE email='$email' and senha='$senha'");

            if($sql->rowCount() > 0){
                $dado = $sql->fetch();

                $_SESSION['id'] = $dado['id'];

                header("Location: index.php");

            } else {
                print 'Usuário e/ou senha incorretos.';
            }
        }catch(PDOException $e){
            print 'error: '.$e->getMessage();
        }
    }
?>

<form method="POST">
    E-mail: <input type='email' name='email'>
    Senha: <input type='password' name='senha'>
    <input type="submit" value='submit'>
</form>