PHP hiba a bejelentkezesnel?

<?php

$username = $_POST[username];

$password = $_POST[password];

$kapcsolat = mysql_connect ('localhost', 'root', '');

mysql_select_db('marci');

$parancs = "SELECT * FROM marci WHERE username = '$username";

$valasz = mysql_query($parancs);

$sor = mysql_fetch_array($valasz);

if ($password == $sor[password])

{

print "You logged in, successfully!";

}

else

{

print "Your username or password is wrong!";

}

mysql_close($kapcsolat);

?>