According to RFC 7231 § 3.1.1.5,[1] a POST request that does not include a
Content-Type header may be interpreted by the server in one of two ways:
1. It may assume application/octet-stream. In this case, PHP and the
Action API will not see the request as having any parameters, and so
will probably serve the auto-generated help page.[2]
2. It may "sniff" the content type. It's likely enough to correctly
guess application/x-www-form-urlencoded in this case, and therefore PHP and
the Action API will see the request as having the intended parameters.
It turns out that HHVM and PHP 7 (at least as used at Wikimedia) differ in
their behaviors: PHP 7 seems to choose option 1, while HHVM chooses option
2.
Thus, clients that have been generating POST requests to Wikimedia wikis'
Action APIs without a Content-Type header will have been receiving expected
results from HHVM but will now start receiving unexpected results as
Wikimedia's migration to PHP 7 proceeds.[3] Affected clients should be
updated to include the Content-Type header in their requests.
See https://phabricator.wikimedia.org/T230526 for some details on this
issue.
[1]: https://tools.ietf.org/html/rfc7231#section-3.1.1.5
[2]: As seen for example at https://www.mediawiki.org/w/api.php.
[3]: See https://phabricator.wikimedia.org/T176370 for progress on the
migration.
--
Brad Jorsch (Anomie)
Senior Software Engineer
Wikimedia Foundation