returning php session to ajax - php

Question

I am having trouble getting jQuery ajax to recognise a session. I am creating a php session from a login script, but what is happening is that when ajax loads the authenticated page, the session is always unset. For example, in the secure page, if I refresh the page, the session id changes each time. I have session_start(); in each page. Can someone please show me the correct way to handle sessions with ajax and php? I have spent 2 days and have used google so much, I will probably get an invite to there xmas lunch :-) I have included the relevant code and would be grateful for any help. Thanks
PS. If it makes any difference, I am trying to develop mobile app using jquery mobile.
login html js
$(function() {
$("#kt_login1").click(function() {
var user = $('#user').val();
var pass = $('#pass').val();
if (user == '') {
$("#login_message").html('This field cannot be empty')
$('label[for=user]').addClass("label")
return false;
}
else if (pass == '') {
$("#login_message").html('This field cannot be empty')
$('label[for=pass]').addClass("label")
return false;
}
else $('label[for=user]').removeClass("label");
$('label[for=pass]').removeClass("label");
//alert(user + pass + ok);
data = 'user=' + user + '&pass=' + pass;
$.ajax({
type: "POST",
url: "testajax.php",
cache: false,
data: data,
success: function(data) {
if (data == 'authenticated') {
//alert(user);
document.location = 'secure.php';
}
else $('#login_message').html('You are not authorised');
//$(ok).val('Logged In');
//$("#login").get(0).reset();
//$("#form").dialog('close');
},
error: function(xhr, ajaxOptions, thrownError) {
jAlert('There was an exception thrown somewhere');
alert(xhr.status);
alert(thrownError);
}
});
return false;
});
});
testajax.php
<?php
// test wether the user session is already set
$username = mysql_real_escape_string($_POST['user']);
$pass = mysql_real_escape_string(md5($_POST['pass']));
mysql_connect('localhost', 'root', '');
mysql_select_db('sample');
//now validating the username and password
$sql="SELECT * FROM user_usr WHERE username_usr='$username' and password_usr='$pass'";
$result=mysql_query($sql);
$row=mysql_fetch_array($result);
//if username exists
if(mysql_num_rows($result)>0) {
session_start();
$_SESSION['u_name']=$row['name_usr'];
/*
echo '<pre>';
print_r( $_SESSION['u_name'] );
print_r( $_REQUEST );
echo '</pre>';
exit;
*/
echo 'authenticated';
}
else
{
echo 'Unknown User';
}
?>
+++++SOLUTION+++++
Changed form input from submit to button and voila. All ok

Answer 1

you have to call session_start() each time working with a session (not only when creating it)
see: http://php.net/manual/en/function.session-start.php

Answer 2

There are a whole load of reasons this might be the case - but you're code is upside down! This may well be the cause (or a contributory factor).
You should not treat the existence of a session as evidence of authentication (the contents of the session is another thing altogether). Call session_start() at the top of your script - not conditionally, half-way through.
As to why your session data is not available...that's an FAQ here - have a look at some of the previous answers for potential causes / how to investigate.