sessions not being set in all pages on php5 - php

Question

I am using session_start(); at the top of my login page. After a user logs in, a message is displayed on screen which shows that the session is being set. But, I cannot carry sessions from page to page or can I echo out SID. It is a blank value. I would be grateful if someone could show me where I am going wrong. Thanks
<?php
$userpost = mysql_real_escape_string($_POST['user']);
if (!isset($_SESSION['user'])) {
session_start();
$_SESSION['user'] = $userpost;
}
echo $_SESSION['user'] .' '. 'Just logged in' . SID;
// Or maybe pass along the session id, if needed
?>

Answer 1

You have to have session_start(); on the very top of your code, after <?php. Since you are checking if the session is set without starting the sessions, your code will fail.
Is has to be like this:
<?php
session_start();
$userpost = mysql_real_escape_string($_POST['user']);
if (!isset($_SESSION['user'])) {
$_SESSION['user'] = $userpost;
}
echo $_SESSION['user'] .' '. 'Just logged in' . SID;
// Or maybe pass along the session id, if needed
?>

Answer 2

It's because you're always looking in $_POST for your user data.
Bring the session_start() out of that condition:
<?php
session_start();
$userpost = mysql_real_escape_string($_POST['user']);
if (!isset($_SESSION['user'])) {
$_SESSION['user'] = $userpost;
}

Answer 3

You said that you called session_start() at the top of your login page, but you did not mention your other pages. session_start() needs to be called at the top of every page in your application. I generally put my session_start() logic, along with a snippet of code for logging the user out after a period of inactivity, in an include file and then include it at the top of every page.
<? session_start();
if (isset($_SESSION["last_activity"]) && (isset($_SESSION["username"])) && ((time() - $_SESSION["last_activity"]) > 900))
{
unset($_SESSION["username"]);
}
else
{
$_SESSION["last_activity"] = time();
}
?>