PHP Login Script Session not surviving

Question

I'm having issues with a simple login system that I'm trying to create. Here is my setup:

index.php

<?php if(!isset($_SESSION['username'])){
session_start();
//present login form with action=login.php
} else {
//display actual page
}
?>

login.php

<?php
if(!empty($_SESSION['username'])){
    echo $_SESSION['username'];
} else {
$username=$_POST['username'];
$password=sha1($_POST['password']);
$link=mysqli_connect("localhost","root","password","database",3306,"/tmp/mysql.sock");
$query="SELECT * FROM login WHERE `name`='".$username."' AND `passwordhash`='".$password."' LIMIT 1";
$result=mysqli_query($link,$query);
if(mysqli_num_rows($result)==1){
            session_start();
    $_SESSION['username']=$username;
    $row=mysqli_fetch_assoc($result);
    $_SESSION['firstname']=$row['firstname'];
    $_SESSION['lastname']=$row['lastname'];
    echo "Username: ". $_SESSION['username']." Name: ".$_SESSION['firstname']." ".$_SESSION['lastname']; //this is just here as POC. If I redirect to index.php (using header()), it shows the login form (claims $_SESSION['username'] is unset).
}
}
?>

Once I log in, it shows my username and first and last names just fine on login.php (not a database problem). But if I redirect to index.php, it shows the login form immediately. What's the issue? EDIT: Even after adding session_start() to login.php, the same outcome results.

Answer 1

使session_start（）成为文件的第一行，它必须包含在使用会话变量的所有文件中