Description:
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It appears Pipelining support (though the CONSTANT exists) does not behave as the user would expect.

Came across this issue here:
  http://stackoverflow.com/questions/24599720/how-to-send-php-curl-handles-through-a-single-tcp-socket-using-http-pipelining

Did some testing of my own, got same results as the person who submitted above StackOverflow question.

Looked a bit further and found this:
  http://curl.haxx.se/mail/lib-2014-06/0219.html

Looked in source code for the curl extension and did not see any evidence that the named CONSTANTS are available in the PHP implementation.

Test script:
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The stackoverflow question points to the following test script:

https://github.com/anovikov1984/pipelining-example/blob/master/pipeline.php

Expected result:
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netstat feedback should show that only 1 connection (or up to some MAX#) was made during the curl_multi_* session

netstat -t -u -c

Actual result:
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based on the script provided, the netstat feedback shows 3 connections were made during the curl_multi_* session.