无法从$ _POST变量输入数据，但如果我从其工作中输入

Question

I want to ask.. why my $_POST data can't pass to parameters, but if I input from this controller data is inserted, I just don't know what to do.. and if I send from my html its return false, here is my code

class NilaiController{
var $c;
function __construct() {
    $this->c=new Nilai();
}
function insert($nis,$namaSiswa,$namaGuru,$mapel,$tahunAjaran,
        $semester,$kategori,$nilaiUjian,$nilaiTugas,$kehadiran,$kelas){

    $ins=$this->c->insertNilai($nis, $namaSiswa, $namaGuru, $mapel, $tahunAjaran,
            $semester, $kategori, $nilaiUjian, $nilaiTugas, $kehadiran, $kelas);
    if($ins){
         echo var_dump($ins);
    }
    else{
        echo var_dump($ins);
    }
}
$n=new NilaiController();

$in=INPUT_POST;
$nis= $_POST['NIS'];
$namaSiswa= filter_input($in, 'Nama_Siswa');
$namaGuru= filter_input($in, 'Nama_Guru');
$mapel= filter_input($in, 'Mapel'); 
$tahunAjaran= filter_input($in, 'Tahun_Ajaran');
$semester= filter_input($in, 'Semester');
$nilaiUjian= filter_input($in, 'Nilai_Ujian');
$nilaiTugas= filter_input($in, 'Nilai_Tugas');
$kehadiran= filter_input($in, 'Kehadiran');
$kategori=filter_input($in,'kategori');
$kelas= filter_input($in, 'Kelas');
f($nis!=NULL){
 $obj=$n->insert($a,$namaSiswa,$namaGuru,$mapel,$tahunAjaran,
        $semester,$kategori,$nilaiUjian,$nilaiTugas,$kehadiran,$kelas);
if($obj){
    //echo var_dump($obj);
}
else{
   // echo var_dump($obj);
}
 }
    else{
        echo "null";
       }

and this is my query

class Nilai{
var $db;
public function __construct() {
    $this->db=new Koneksi();
}
public function insertNilai($nis,$namaSiswa,$namaGuru,$mapel,$tahunAjaran,
        $semester,$kategori,$nilaiUjian,$nilaiTugas,$kehadiran,$kelas){
    //ini buat user dengan Role Guru aja
    $query="INSERT INTO Data_Nilai (NIS,Nama_Siswa,Nama_Guru,Mapel,"
            . "Tahun_Ajaran,Semester,kategori,Nilai_Ujian,Nilai_Tugas,"
            . "Kehadiran,Kelas) "
            . "VALUES(?,?,?,?,?,?,?,?,?,?,?)";
    $stmt= mysqli_prepare($this->db->connect(), $query);
    mysqli_stmt_bind_param($stmt,'sssssssiiis',$nis,$namaSiswa,$namaGuru,$mapel,$tahunAjaran,
        $semester,$kategori,$nilaiUjian,$nilaiTugas,$kehadiran,$kelas);
    if(mysqli_stmt_execute($stmt)){
        return TRUE;
    }
    else{
        return FALSE;
    }
}

this function return true if i input values from inside not from $_POST variabel..