为什么$ _SESSION没有按预期保存我的变量？ [重复]

I am new to PHP and mySQL databases. I am trying to build an android app for a university-project, that pulls data from a mySQL MAMP database.

I have a working connection to the database and i can successfully log in. Since the user-name is a unique key, i want to give it to another .php file, that pulls user specific data from another table of the database. For some reason $_SESSION does not work. Have been trying things for a few hours now - can't seem to fix it.

Looking forward to ANY help i can get. Thanks a lot :)

EDIT: If i manually type in the user-name instead of the variable $snr, it works as intended. Thats why i think it has something to do with $_SESSION in showimpfungen.php

UPDATE: So after i have done some troubleshooting - the problem seems to be the following. If i press the "login" button on my android emulator - a session file is created in which the variable is stored correctly. But after i run the showimpfungen.php script, another session file is created with a different ID. I guess thats the reason for Notice: Undefined index: why in /Applications/MAMP/htdocs/showimpfungen.php on line 11. Also, i get PHP Notice: Undefined index: user_name in /Applications/MAMP/htdocs/login.php on line 7 and PHP Notice: Undefined index: password in /Applications/MAMP/htdocs/login.php on line 9 when running it in browser. My guess is, that i get those because they get their values from the user input in the emulator.

conn.php:

<?php

session_start();


$db_name = "Impfdatenbank";
$mysql_username = "root";
$mysql_password = "root";
$server_name = "localhost";

$conn = mysqli_connect($server_name, $mysql_username, $mysql_password, $db_name);

if(mysqli_connect_error($conn))
{
    echo "Failed to connect";
}

?>

login.php:

<?php

session_start();

require "conn.php";


$user_name = $_POST["user_name"];
$_SESSION['why'] = $user_name;
$user_pass = $_POST["password"];

$mysql_qry = "select * from personen_daten where svnr like '$user_name' and password like '$user_pass';";

$result = mysqli_query($conn, $mysql_qry);


if(mysqli_num_rows($result) > 0){

    echo "Login successful.";
}

else{
    echo "Login failed.";
}


?>

showimpfungen.php:

<?php

session_start();

require "conn.php";

$snr = $_SESSION['why'];

$mysql_qry = "select * from Impfungen where svnr like '$snr'";

$query = mysqli_query($conn, $mysql_qry);


if($query)
{
    while ($row = mysqli_fetch_array($query))
    {
        $flag[] =$row;
    }
    print(json_encode($flag));
}
mysqli_close($conn);

?>

</div>