Description:
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I have noticed the temp directory on the web server is full of files named php*.tmp.
The contents of these files is POST data for various ajax calls (mainly the same group of calls). There isn't a temp file per call. If I watch the directoy I can see most temp files appear and then get deleted. However, for whatever reason, sometimes these files are not cleaned up.
The only way I have been able to reproduce this error is using apache bench to post data (26KB) to a script which uses the shell_exec, exec, popen command:

<?php

shell_exec("dir");

echo "FINISHED";

?>

It will only produce the temp file when there is concurrency and lots of requests for each:

ab -p <post_data_file> -c 3 -n 30 <url_to_script>

Our system does perform calls to popen() but the POST data temp files which are being left behind are not always for calls which invoke this function.

I can produce the issue on 5.6.24 and 5.6.25.
I have tried on the following versions and not been able to reproduce it: 5.4.44, 5.5.25, 7.1

I not beein able to reproduce the issue on a Mac running 5.6.18.