<?php

$username = $_POST['txt1'];
$password  = $_POST['txt2'];

if(empty($username) || empty($password)){
    echo "<script>alert('Llena todos los campos!');</script>";
}
else{
    $tsql = "SELECT * FROM CatUsuarios WHERE U_UsdName='$username' AND U_Password='$password' LIMIT 1";
}
$stmt = sqlsrv_query( $conn, $tsql);
while($Row=sqlsrv_fetch_array($stmt)){
    if($stmt == true){
        $_SESSION['valid_user'] = true;
        $_SESSION['usuario'] = $username;
        header('Location: index.php');
        die();
    }else{
        header('Location: error.html');
        die();
    }
}

?>