$getusername = mysql_query("SELECT username FROM userdata");
//Appears to be OK

$getpassword = mysql_query("SELECT password FROM userdata WHERE username='`$username`'");
//Maybe change to:
$getpassword = mysql_query("SELECT password FROM userdata WHERE username='$username'");


$row = mysql_fetch_assoc($getusername);
if ($username!="$row['username']") header('location:loginpage.php?error=1');
//Maybe change to:
if ($username!=$row['username']) header('location:loginpage.php?error=1');

$row = mysql_fetch_assoc($getpassword);
if ($password!="$row['password']") header('location:loginpage.php?error=1');
//See above