$sql = "select count(*) from usuarios where (username='$user' or email='$user') and password='$contra'";

$result = $con -> query($sql);

$row = $result -> fetch_assoc();

if(???????== 0){

    echo "<h1 style='text-align:center'>Ingreso invalido del sistema</h1>";
    echo "<br>";
    echo "<a style='text-align:center' href='index.html'>Volver a Intentarlo</a>";

}

else {}