会话集但不访问其他页面

Here's my login code where I set the session:

<?php

session_start();

foreach ($query_result as $rows) {
    if (($rows['admin_username'] == $username) && ($rows['admin_password'] == $password)) {
        $_SESSION['valid'] = true;
        $_SESSION['timeout'] = time();
        $_SESSION['username'] = $username;
        $match_flag = true;
        break;
    }
}

And it is my next file code where session is not accessible:

<?php
    session_start();
    echo " session value username is: ";
    var_dump($_SESSION['username']); exit();
?>

It works perfectly on localhost but when I run it on the server, it will print NULL in session values.

Please help me regarding this issue.

Use error reporting and check if it shows any error.

ini_set('display_errors', '1');    
error_reporting(E_ALL);