How do I assign a string to $_SESSION?

I've fixed this, thank you everyone for your help!

The value of $_SESSION["nama_pengguna"] and $_SESSION["kata_laluan"] remains NULL and doesn't change to $nama_pengguna and $kata_laluan after passing all 3 if statements.

when:

$_POST is not NULL
$_POST["daftar"] is not NULL
$result is not NULL

value I get:

$_SESSION["nama_pengguna"] => NULL
$_SESSION["kata_laluan"] => NULL

expected value:

$_SESSION["nama_pengguna"] => $nama_pengguna
$_SESSION["kata_laluan"] => $kata_laluan

extra information:

$_SESSION["login"] works fine
- when passes all if statements, $_SESSION["login"] => true
- when doesn't pass all if statements, $_SESSION["login"] => false
- I expected $_SESSION["nama_pengguna"] and $_SESSION["kata_laluan"] to copy this behaviour but it doesn't.
I've tried removing the first appearance of $_SESSION["nama_pengguna"] and $_SESSION["kata_laluan"], but those two variables stopped existing once I did that.

snippet from login.php

<?php
    session_start();
    include("sambungan.php");
    $_SESSION["login"] = false;
    $_SESSION["nama_pengguna"] = NULL; #Problem
    $_SESSION["kata_laluan"] = NULL;  #Problem

    if (!empty($_POST) && !empty($_POST["nama_pengguna"]) && !empty($_POST["kata_laluan"])) {
        $nama_pengguna = $_POST["nama_pengguna"];
        $kata_laluan = $_POST["kata_laluan"];
    
        if (isset($_POST["daftar"])) {
            $sql = "INSERT INTO urusetia (nama_pengguna,kata_laluan) VALUES ('$nama_pengguna','$kata_laluan')";
            $result = mysqli_query($sambungan,$sql);
            if ($result) {
                $_SESSION["login"] = true;
                $_SESSION["nama_pengguna"] = $nama_pengguna; #Problem
                $_SESSION["kata_laluan"] = $kata_laluan; #Problem

                $_POST = array();
                header("Location:./hakim.php");
                die();
            }
        }
    ...

snippet from login.php

...
        <form action="login.php" method="post">
            <table>
                <tr>
                    <td>Nama Pengguna</td>
                    <td><input type="text" name="nama_pengguna" autocomplete="off" placeholder="max 30 characters" required></td>
                </tr>
                <tr>
                    <td>Kata Laluan</td>
                    <td><input type="password" name="kata_laluan" autocomplete="off" placeholder="max 15 characters" required></td>
                </tr>
            </table>
            <input type="submit" name="daftar" value="Daftar">
            <input type="submit" name="log_masuk" value="Log Masuk">
        </form>
...

echo-ing all the $_SESSION variables

Solution

You checked just $_POST in first condition. it is good practice to check if $_POST["nama_pengguna"] and $_POST["kata_laluan"] is not empty too. (isset is not enough):

...
if ($_POST && !empty($_POST["nama_pengguna"]) && !empty($_POST["kata_laluan"])) {
    // not you can be sure both of that data is exist and have some data.
    $nama_pengguna = $_POST["nama_pengguna"];
    $kata_laluan = $_POST["kata_laluan"];

    ....
} else {
  // return some error because of being empty needed data.
}

but you maybe you have to add an "else" to you condition for return some errors because the data does not have enough data we need.