This is currently the failure for the latest Travis CI build with hhvm+mysql:

https://travis-ci.org/wikimedia/mediawiki/jobs/148367595

Warning: Invalid argument: function: not a valid callback array in /home/travis/build/wikimedia/mediawiki/tests/phpunit/includes/session/PHPSessionHandlerTest.php on line 183
Warning: Session callback expects true/false return value in /home/travis/build/wikimedia/mediawiki/tests/phpunit/includes/session/PHPSessionHandlerTest.php on line 183
Fatal error: Failed to initialize storage module: user (path: /var/lib/hhvm/sessions) in /home/travis/build/wikimedia/mediawiki/tests/phpunit/includes/session/PHPSessionHandlerTest.php on line 183
Warning: Invalid argument: function: not a valid callback array
Warning: Session callback expects true/false return value
Warning: Destructor threw an object exception: exception 'MWException' with message 'MediaWiki\Session\PHPSessionHandlerTest::tearDown() must call parent::tearDown()' in /home/travis/build/wikimedia/mediawiki/tests/phpunit/MediaWikiTestCase.php:118
Stack trace:
#0 (): MediaWikiTestCase->__destruct()
#1 {main}

The true/false thing could be the version detection in f9d07f7ff23861c7abca27b72f0be29c292aa357 failing (or could just be a cascade from the previous error). The other errors don't make any sense to me. HHVM issue #5072 maybe?

I see the log linked was generated on July 29, 2016, while f9d07f7f that fixed the true/false thing was merged in October.

I think Gergő is right about the other error, since I see that log is using HHVM 3.6.6 while that HHVM bug wasn't fixed until 3.7.

Change 339096 had a related patch set uploaded (by Krinkle):
build: Update HHVM for Travis to 3.12

https://gerrit.wikimedia.org/r/339096

Change 339096 merged by jenkins-bot:
build: Update HHVM for Travis to 3.12

https://gerrit.wikimedia.org/r/339096

In T75175#3048735, @gerritbot wrote:

Change 339096 merged by jenkins-bot:
build: Update HHVM for Travis to 3.12

https://gerrit.wikimedia.org/r/339096

This didn't work as expected since the hhvm-3.12 option is only available on trusty images from Travis CI. We're on the default, which is apparently precise still.

I tried trusty on my GitHub fork, but ran into some unrelated issues. Tracking at https://github.com/travis-ci/travis-ci/issues/7368.

Change 339345 had a related patch set uploaded (by Krinkle):
build: Change Travis image from precise to trusty (Fix HHVM build)

https://gerrit.wikimedia.org/r/339345

Change 339345 merged by jenkins-bot:
build: Change Travis image from precise to trusty (Fix HHVM build)

https://gerrit.wikimedia.org/r/339345