Description:
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If you enable sessions on a page that runs any program execution command and do two or more concurrent requests for the page, Apache immediately stops responding and sits doing nothing (using 0% of the CPU) until you close it.

I'm using a clean install of both Apache and PHP for windows; both are using their default configurations. This was tested under Windows 2000 and XP using Apache 1.3.33. The problem seems to be the same under Apache 2.x as well.

There is a workaround for this problem, and that is to call session_write_close before calling any program execution function.

Note: This seems to be a Windows specific problem. This was tested with a *NIX copy of PHP and it worked just fine.

Reproduce code:
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http://www.cs.rit.edu/~ndk4275/multi.php.txt

Expected result:
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When the page loads, click on "Test". It will open two small windows and will wait for 10 seconds before executing the actual test. It might seem like the browser is not responding for the ten seconds before the test, but this is due to the fact that the wait function is a greedy CPU function. When the test starts, the two small browser windows will load the test page with the program execution function at the same time and the main window will reload the current page. At this point, Apache should stop responding. Attempting to make any request to the server will result in an indefinite wait.

Actual result:
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After the test executes, Apache stops responding.