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

My problem was on server-side the session.save_path not definied to my server. Thanks a lot to all, who helped me :)