<?php

$link = mysql_connect("localhost","root","chiave") or die("Impossibile connettersi a mysql");
echo "Connessione mysql stabilita<br>";
$db = mysql_select_db("my_db", $link) or die("Impossibile connettersi a my_db");
echo "Connessione my_db stabilita<br>";


$username = mysql_real_escape_string(( $_POST["username"] );
$password = md5( mysql_real_escape_string( $_POST["password"] ) );

$query= "SELECT * FROM  members WHERE username='$username' and password='$password'";
$result=mysql_query($query, $db) or die ("Impossibile eseguire la query");

$count=mysql_num_rows($result);

if($count==1) { <?

here goes your html code
<?php } else { ?>
and here goes the "you did wrong"
<?php }

mysql_close($link); // you want to close the connection
?>


?>