<form id="form1" name="form1" method="post" action="index.php">
<p>Nome: <input name="email" type="text"> </p>
<p>Senha: <input type="text" name="senha"> </p>

<input type="submit" name="post_back" id="button" value="Submit">
</form>

<?php

session_start();

require("config.php");

if(isset($_REQUEST["post_back"])){

    $tb = $conn->prepare("select id_usuario from usuario where email_usuario=:email and senha_usuario=:senha");
	 $salt = $_POST["senha"];
	 $custo = '08';
	 $hash = crypt($senha, '$2a$' . $custo . '$' . $salt . '$');

    $tb->bindParam(":email", $_POST["email"], PDO::PARAM_STR);
     $tb->bindParam(":senha", $hash, PDO::PARAM_STR);
    $tb->execute();
    $l = $tb->fetch(PDO::FETCH_ASSOC);
    $tb = null;
    if(!empty($l)){

        $_SESSION["usuario"] = $l["id_usuario"]; header("Location: dados.php");

    }else{

    echo "<script>alert('Login Falhou');</script>";

    }

}