Description:
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While two PHP scripts (or two instances of the same script) are running, Apache (2.2.4) stops responding to any new requests (even for non-php pages) until one of them finishes.

It doesn't seem to matter what the scripts are actually doing; the same problem occurs when they're doing any of the following:

-performing calculations (e.g. counting from 1 to 10000000)
-blocking on socket functions
-sleep() ing

Reproduce code:
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wait.php:
<?php sleep(15); ?>

1. Open two browser windows and point them both to wait.php so that they're running at the same time.
2. Before they finish, open a third browser window and point it to any other URI on the server (even a non-php page).


Expected result:
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The third window should load immediately.

Actual result:
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The third window does not load until one of the two PHP scripts finshes.

Note: If the third request is for a PHP page containing an error_log() at the very beginning, then the logfile output is not generated as long as the first two pages are running (suggesting that Apache isn't getting around to starting PHP during this time).