Description:
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(Background: curl_easy_setopt(3) supports a CURLOPT_PRIVATE that takes a char* and does nothing with it. It can then be retrieved with curl_easy_getinfo(3) and CURLINFO_PRIVATE -- cURL >=7.10.3)

While the two CURL*_PRIVATE constants aren't mentioned in any detail in the PHP manual, they are listed in cURL's predefined constants page, and using (eg) an array for the value results in an array->string conversion warning, so I guess there's at least a little bit of support somewhere.

However using them gives buggy results - mostly binary junk.

Test script:
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$curl = curl_init("http://www.example.com");
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_PRIVATE, "123"); // *
curl_exec($curl);

var_dump(curl_getinfo($curl, CURLINFO_PRIVATE));

// * mostly happens with numbers; try different string lengths

Expected result:
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string(3) "123"

Actual result:
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string([varies]) "[binary junk]"