How do I sync my users connections PHP - php

Question

I have a class in PHP called cUser:
class cUser {
var $m_email;//The users email adresse(String)
var $m_password;//His password(String)
var $m_username;//His username(String)
var $m_active;//If the user have been activate (By following a link send to him via is email)(Bool)
function __construct($p_username, $p_password, $p_email, $p_active) {
$this->m_username = $p_username;
$this->m_password = $p_password;
$this->m_email = $p_email;
$this->m_active = $p_active;
}
//this is the important part...
function connexion() {
include "Config.php";//include all the parameters needed to connect to the DB
$cn = new cConnexion($ConnexionDBHost, $ConnexionDBName, $ConnexionDBLogin, $ConnexionDBPassword);//Initiate a connection to the DB
if($cn->DBConnexion())//If it is connected {
$parameters = array('username'=>$this->getUsername(), 'password'=>$this->getPassword());//create an array with the username and the password
$getConnexion = $cn->SecureSelect("SELECT username, password, email, active FROM user WHERE BINARY username = :username AND BINARY password = :password", $parameters);//selecte the user in the DB (for DB description see below code)
if($getConnexion != null) { //if there is no error in the query.
$resultSet = $getConnexion->fetch();//fetch the results
if($resultSet != null) { //if there is a match
//assigne the DB field values to this instance of cUser
$this->setUsername($resultSet['username']);
$this->setPassword($resultSet['password']);
$this->setEmail($resultSet['email']);
$this->setActive($resultSet['active']);
if($this->getActive() == 1) {
//If the user has been activate already return success
}
else {
//Else send an activation email to the user.Dont connecte him and return an error message
}
}
else {
//Send an error message
}
}
else {
//send an error message
}
}
else {
//send an error message
}
}
//this are not important for the question but I put them there so you can see what kind of operation the class is doing.
function delete(){//Delete this instance of cUser from de DB}
function insert(){//Insert this instance of cUser from the DB}
function update($p_email, $p_username, ...){//Update this instance of cUser with the new parameters}
function activateAccount(){//Activate this instance of cUser}
//And all the getters and setters associate with the class attributes.
}
Here is the MySQL Table containing the field for the cUser class (roughly coded):
USER
varchar email,
varchar password,
varchar username,
tiny int activate,//1 or 0
tiny int connected//1 or 0
Question:
How can I implement or change the function connection so one instance of a user is connected at the same time?
Note:
I already know I can just check if the DB connected field is set to 1 but if two user access the DB at the same time it would create a problem (race condition or something like that).
Is there something like a mutex or semaphore I can use to sync the access of the DB field connected??
Example:
David fill the HTML form and submit it with user name and password ("Dav1", "ThisIsPassword"), a process page create the cUser instance and connect to check if Dav1 already existe then give him access to the rest of the web-app.
Now Davos fill the form and submits it with the same user-name and password that David used because Davos and David are friend and they shared the same account and shared there password.
With the existing code both David and Davos can then access the web application at the same time with the same account what I want is that when David connect Davos get an error message that tell him that the user is either already connected or the user-name/password doesn't match.

Answer 1

Use a transaction.
In MySql you can also use SELECT FOR UPDATE statement.
Pseudocode:
$transaction = db->beginTransaction();
try {
$user = User::getByUsername($username);
if ($passwordImcorrect)
throw new Exception('invalid credentials');
if (user->loggedIn)
throw new Exception('already logged in');
user->loggedIn = 1;
user->save();
$transaction->commit();
}
catch (Exception $e) {
echo $e->getMessage();
$transaction->rollback();
}