php session variable randomly changes - php

Question

I have a website that has a membership system. When users log in, I validate username/password from the database and start a session, $_SESSION['userid'] that contains their id (I have not implemented anything using cookies yet)
I have a problem, the system works fine most of the times, but some users have reported that they eventually find themselves logged in to some other random users account. That probably means that $_SESSION['userid'] changes without any reason to something else and I'm pretty sure I'm not doing anything to change it.
Any ideas why this could be happening ?
edit : Summary of what I am doing
This method start the session
function startSession($id){
$_SESSION['logged_in'] = 1;
$_SESSION['userid'] = $id;
}
This method checks login
function isLoggedIn(){
return isset($_SESSION['logged_in']) && isset($_SESSION['userid']) && $_SESSION['userid']!=" " && $_SESSION['logged_in']==1;
}
This is the logout method
function logout(){
$_SESSION['logged_in'] = 0;
$_SESSION['userid'] = 0;
unset($_SESSION['logged_in']);
unset($_SESSION['userid']);
session_destroy();
if (!isLoggedIn()){ return "S3"; }
else { return "E3"; }
}
And this is how I check if the user is logged in most places
if (isLoggedIn()){ $profileid = $_SESSION['userid']; }
This is the login function, this is where I call startSession
function login($username, $password){
$pdo = newPDO();
$username = sanitize_string($username);
$password = sha1(sanitize_string($password));
$query = $pdo->prepare("SELECT id FROM ".TABLE_PROFILE." WHERE nick=:nick AND pass=:pass LIMIT 1");
$query->execute(array(':nick'=>$username, ':pass'=>$password));
$result = $query->fetch(PDO::FETCH_ASSOC);
if (count($result['id']) == 1){
startSession($result['id']);
loginExecution();
return "S1";
}
else{ return "E1"; }
}

Answer 1

The problem is in your login function
Your script is not checking username and password for any data, and if the username and password is empty or incorrect, your client will gets the firs available id from database.
function login($username, $password){
$pdo = newPDO();
$username = sanitize_string($username);
$password = sanitize_string($password);
// Check data for existing
if (empty($username)) throw new Exeption('Empty username');
if (empty($password)) throw new Exeption('Empty password');
$password = sha1($password);
$query = $pdo->prepare("SELECT id FROM ".TABLE_PROFILE." WHERE nick=:nick AND pass=:pass LIMIT 1");
$query->execute(array(':nick'=>$username, ':pass'=>$password));
$result = $query->fetch(PDO::FETCH_ASSOC);
if (count($result['id']) == 1){
startSession($result['id']);
loginExecution();
return "S1";
}
else{ return "E1"; }
}
P.s. Always check incoming data, before SQL queries

Answer 2

Realistically the only thing I can think of that would cause this is something setting $_SESSION['userid'] to another members id. Assuming this is what you are using to check which members information to show. Are you perhaps doing something if the $_SESSION['userid'] variable is not set that may end up setting $_SESSION['userid'] incorrectly? i.e. if their php session is reset.