erro ao validar

galera to estudando sessoes porem nao to conseguindo verificar se o usuario existe na tabela pra poder pegar a variavel de sessao e trabalha com ela segue o codigo que eu to testando:

<?php

$host="localhost";
$user="root";
$pass="";
$banco="clinica";

$conn=mysql_connect($host,$user,$pass) or die();
$db=mysql_select_db($banco,$conn) or die();

if (!empty($_POST) AND (empty($_POST['usuario']) OR empty($_POST['senha']))) {
header("Location: logar.php");
exit;
}

$usuario=addslashes($_POST['usuario']);
$senha=addslashes($_POST['senha']);

session_start();

$sql="SELECT * FROM usuarios WHERE usuario ='$usuario' AND senha='$senha' LIMIT 1";

$query=mysql_query($sql) or die(mysql_error());
$resultado=mysql_fetch_array($query);

if($query){
echo "logou<br>";
$_SESSION['usuario']=$resultado['usuario'];
echo "Ola,".$_SESSION['usuario'];
}

?>

Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0