md5 error

<?php
include "koneksi/koneksi.php";
session_start();

// untuk mencegah sql injection
function anti_injection($data){
    $filter=mysql_real_escape_string( stripslashes ( strip_tags ( htmlspecialchars($data, ENT_QUOTES))));
    return $filter;
}
$username = anti_injection($_POST['username']);
$password = anti_injection(md5($_POST['password']));

//pastikan username dan password berupa huruf atau angka
$cek = mysql_query("SELECT * FROM siswa WHERE username='$username' and password='$password'");
$id = mysql_fetch_array($cek);
if(mysql_num_rows($cek)>=1){
            @$_SESSION['username'] = $id['username'];
            @$_SESSION['password'] = $id['password'];
              header('location: user');
}else{
echo "<script>alert('Username atau password salah!'); document.location.href=\"index.php\"</script>";
        }
?>