login

index.php

<?php

session_start();

if(isset($_SESSION['id']) && empty($_SESSION['id']) == false) {
    echo "Área restrita...";
} else {
    header("Location: log.php");
}
?>

login.php

<?php

session_start();

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

    $dsn = "mysql:dbname=log;host=127.0.0.1";
    $dbuser = "root";
    $dbpass = "";

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

        $sql = $db->query("SELECT * FROM usuario WHERE email = '$email' AND senha = '$senha'");

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

            $dado = $sql->fetch();

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

            header("Location: index.php");

        }

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

?>

Página de login

<form method="POST">

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

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

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

</form>