Description:
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Hello. A mistake is noticed when you try paying for the session while working with it at this point.
I'm from Russia, therefore, to explain through an interpreter is not the best option, I will show you some example code, and you can understand the result of work.

Reproduce code:
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Example source code: http://www.damagelab.org/dl/scripts/phpbuginsession.zip

zip.php - test file for sleep on wrok with session.
ajax.php - interface for example testing, include 2 ajax query.
ajax.js - library for used ajax.php
log.txt - debug log with result in real-time working

Expected result:
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He must show for every ajax request(ajax.php?do=status):
46:[00:26:46] work 2
47:[00:26:46] work 2
48:[00:26:46] work 2
49:[00:26:49] work 3


Actual result:
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52:[00:26:52] end

This finding does not immediately, but only when the script is run for zip.php and the latest result.