I think your problem is that you have to enter an 'else' statement, otherwise it will just show the only value it knows, for example:
PHP Code:
if (strpos($username, ****) === true)
{
echo "That does not appear to be a valid username, please try again.";
include 'index.html';
}
else
{
echo "enter w/e you want to tell them that they have a valid username here";
}