What is Wrong with This Script?

[Seraphino]

<?
$hostname="<HIDDEN>";
$username="damasterjo";
$password="<HIDDEN>";
$database="damasterjo";

mysql_connect($hostname,$username,$password);
@mysql_select_db($database) or die( "Unable to select database");

$query="SELECT * FROM Customers";
$result=mysql_query($query);
$num=mysql_numrows($result);

//Checks If The Username and Password Exists
//Declaring Variables
$usernameisvalid=0;
$passwordisvalid=0;
$passwordexists=0;
$usernameexists=0;
//loop through all the entrys in the database to see if any match...
    while ($usernameisvalid < $num) {
        //set the username they want to a variable
        $checkuser=$_POST['Username'];
        //Set the database user name into a variable
        $existing=mysql_result($result,$usernameisvalid,"Username");
        //Check whether or not they are the same

            if  ($checkuser == $existing)
            {
                $usernameisvalid=$num;
                $usernameexists=1;
            }
                $usernameisvalid++;
            }

    while ($passwordisvalid < $num) {
        //Set Password They Want Into A Variable
        $checkpassword=$_POST['Password'];
        //Set the database password into a variable
        $existingpass=mysql_result($result,$passwordisvalid,"Password");
        //Check whether or not they are the same
        
            if ($checkpassword == $existingpass)
            {
                $passwordisvalid=$num;
                $passwordexists=1;
            }
                $passwordisvalid++;
            }
    
    if ($passwordexists == 0 and $usernameexists == 0)
    {
        echo "This Username or Password is Invalid!";
    }
    if ($usernameexists == 1 and $passwordexists == 1)
    {
        echo "This Username and Password Exists!";
    }
?>

For some reason, no matter what Username or Password I enter, it keeps saying that it is valid.

Btw, just to let everyone know, this is for a joined project between me and Damasterjo.