<?php
session_start();

if(isset($_POST['email']) && empty($_POST['email']) == false){
    $email = addslashes($_POST['email']);
    $senha = md5 (addslashes($_POST['senha']));

    $dsn="mysql:dbname:blog,host=localhost";
    $dbuser = "root";
    $dbpass = "";

    try{
        $db =new PDO($dsn, $dbuser, $dbpass);

        $sql = $db->query("Select * from usuarios where email ='$email' and senha ='$senha'");

        if($sql->rowCount() >0){

            $dado = $sql->fetch();

            $_SESSION['id'] = $dado['id'];
            header("Location: index.php");


        }

    }catch(PDOExecption $e){
        echo "Falhou:" .$e->getMessage();
    }

}
?>

Pagina de Login

<form method="POST">

    E-mail:<br/>
    <input type="email" name="email" /><br/><br/>

    Senha:<br/>
    <input type="password" name="password" /><br/><br/>

    <input type="submit"  value="Entrar" />

    </form>