PHP - Session doesnt work on mobile

Question

For login im using a session_start(), for desktop its works fine, but on mobile, it doesnt work. When I make a session_id() with variable, for example, session_id('login') its works on mobile, but break other sessions on other computers. But when session_id() is generated automatically, it doesn't work on mobile. What should I do?

My session_start code on index.php

session_start();
if (isset($_SESSION['username'])){
    session_id();
}

Whole code for login.php file

<?php
require('config.php');
$usernameOK = false; $passwordOK = false;
if (isset($_POST['username']) and isset($_POST['password'])){

        $username = $_POST['username'];
        $password = $_POST['password'];

        $UserQuery = "SELECT username FROM `members` WHERE username='$username'";
        $userTestResult = mysqli_query($connection, $UserQuery);
        $usernameTEST = mysqli_num_rows($userTestResult);
        if($usernameTEST == 1){$usernameOK = true;}

        $PasswordQuery = "SELECT password FROM `members` WHERE username='$username'";
        $result = mysqli_query($connection,$PasswordQuery);
        $row = mysqli_fetch_row($result);

        if (password_verify($password, $row[0])){$passwordOK = true;}

        if($usernameOK == true && $passwordOK == true){
            $_SESSION['username'] = $username;
        }else{
            $error_message = "Incorrect login data";
        }
    }

    if (isset($_SESSION['username'])){
        header("Location: ../");
        exit;
    }else{?>
    <form class="login-form" method="POST">
        <?php if(isset($error_message)){ ?><div class="mini-mes error"> <?php echo $error_message; ?> </div><?php } ?>

        <span class="placeholder">username</span>
        <input type="text" name="username" placeholder="username" required>

        <span class="placeholder">password</span>
        <input type="password" name="password" id="inputPassword" placeholder="password" required>

        <button type="submit">Login</button>
        <a class="form-link" href="register.php">Register</a>
    </form>

<?php } ?>

Answer 1

Not really sure what your code does, because I dont see anything regarding setting the username or anything of that sort. What your code does is, it starts the session and checks if username variable in session is set, and if YES, it calls session_id.

Can you use this to check if the session is created properly?

session_start();
if (session_status() !== PHP_SESSION_NONE)
{
echo session_id();
}

Starts a session and shows you the session ID if the session_status is not PHP_SESSION_NONE