PHP Multiple Concurrent Sessions Per User

The behavior you describe opposes the concept of a browser session. Why would a user want more than one session? Is it a matter of user access controls needing to be enforced? If so, assign users to logical groups and grant permissions to specific groups. Do users need to perform some action on behalf of other users? If so, design the website around that concept instead of trying to create multiple sessions for a single user.

If you really have to do this, you could do something horrible like pass along a query parameter (very insecure!) between pages to act as a session ID, bypassing the actual $_SESSION altogether and managing your own concept of a session. Again, this is not normal and will only lead to headaches/security issues in the future.

Non-Atomic Concurrent Session Management access can be simulated with the following pseudo coded logic:

function main(){
  $locker = new SessionLocking();
  /** read elements the $_SESSION "cached" copy. **/
  $var1 = $_SESSION['var1'];
  $var2 = $_SESSION['var2'];
  /** Pseudo Atomic Read **/
  $locker->lock(); //session is locked against concurrent access.
  $var3 = $_SESSION['var3'];
  $locker->unlock(); //session is committed to disk (or other) and can be accessed by another script.
  /** Psuedo Atomic Write **/
  $locker->lock(); //session is locked against concurrent access.
  $_SESSION['var4'] = "Some new value";
  $locker->unlock(); //session is committed to disk (or other) and can be accessed by another script
}

CLASS SessionLocking {

private static $lockCounter=0;
private static $isLoaded=false;

function __constructor(){
  if (!self::$isLoaded) load();
}

private function load(){
 $this->lock();
 $this->unlock();
}

private function lock(){
  if ($lockCounter<1) try {session_start();} Catch(){}
  $lockCounter++; 
}

private function unlock(){
  if ($lockCount<1) return;
  $lockCounter--;
  if ($lockCounter<1) try {session_write_close();} Catch(){}
}
}

This would be very difficult to do, if at all possible.

Sessions should not have to worry about which tab they are in.

Also, what happens if session 1 in tab 1 opens a new window? Is it a new session?