edit.php

<?php
    include 'lib/library.php';

    if ($_SERVER['REQUEST_METHOD'] == 'POST') {
        $nis                    = $_POST['nis'];
        $nama_lengkap           = $_POST['nama_lengkap'];
        $jenis_kelamin          = $_POST['jenis_kelamin'];
        $kelas                  = $_POST['kelas'];
        $jurusan                = $_POST['jurusan'];
        $Alamat                 = $_POST['alamat'];
        $Golongan_Darah         = $_POST['golongan_darah'];
        $Nama_Ibu               = $_POST['Nama_Ibu'];

        $sql = "UPDATE siswa SET nis = '$nis',
                nama_lengkap = '$nama_lengkap',
                jenis_kelamin = '$jenis_kelamin',
                kelas = '$kelas',
                jurusan = '$jurusan',
                alamat = '$Alamat',
                golongan_darah = '$Golongan_Darah',
                Nama_Ibu = '$Nama_Ibu' WHERE nis = '$nis' ";

            $mysqli -> query($sql) or die  ($mysqli -> error);

            header('location: index.php');
    }

    $nis = $_GET['nis'];

    if (empty($nis)) header('location: index.php');

    $sql = "SELECT * FROM siswa WHERE nis = '$nis' ";
    $query = $mysqli -> query($sql);
    $siswa = $query -> fetch_array();

    if (empty($siswa)) header('location: index.php');

    include 'views/v_tambah.php';
?>