The simplest way to start this topic is, PHPBB is hacked. You must have heard of it by now. And there is an equal chance that you haven't. A community with almost 200.000 human members and 400,000 subscribed users have lost their privacy inside that community and there was nothing PHPBB could do but to put up a foolish status message like this just below.

But the question is, how did the hacker (say script kiddie if you like, but I don't think 90% of the people naming him that can do half of what he did. No glorification though) do it? Here is a step by step description.

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Semaphore is used to restrict access to shared resources, such as shared memory or to limit the number of processes that may simultaneously use a resource, in a multi-process or multi-threaded environment.

In php using semaphore a process must wait while accessing a shared resource currently used by another process. But this may not be required for all cases. Sometimes we need to just verify whether a resource has been locked and move on. Unfortunately we have no way to check the status of a semaphore lock in php. Let's see how we can implement non-blocking semaphore access in PHP.
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