When PHP session handling is involved, concurrent requests to session-started scripts will block each other until the session write is complete. This means that even though the requests are initiated asynchronously, they will be processed one at a time because the session file is locked during each session. Therefore, while the total execution time will be the cumulative time of each script's sleep call (1+2+3=6 seconds), the statements that the requests may be processed out of order and that concurrent requests will be blocked until the session lock is released, are both true.