**If you're reading this and looking for a solution, replace `hhvm` with `hhvm=3.18` in `.travis.yml`**
https://travis-ci.org/wikimedia/IPSet/jobs/491956371 and https://travis-ci.com/wikimedia/css-sanitizer/jobs/176973417 and https://travis-ci.org/wikimedia/php-session-serializer/jobs/491974796
```
$ phpenv global hhvm 2>/dev/null
$ mkdir -p ${TRAVIS_HOME}/.phpenv/versions/hhvm/etc/conf.d
0.86s$ composer self-update
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'array_pop() expects parameter 1 by reference, but the call was not annotated with '&'' in phar://composer.phar/bin/../src/../src/Composer/Util/Silencer.php:51
Stack trace:
#0 phar://composer.phar/bin/../src/../src/Composer/Util/Silencer.php(74): Composer\Util\Silencer::restore()
#1 phar://composer.phar/bin/../src/../src/Composer/Console/Application.php(71): Composer\Util\Silencer::call()
#2 phar://composer.phar/bin/composer(53): Composer\Console\Application->__construct()
#3 /home/travis/.phpenv/versions/hhvm-stable/bin/composer(25): include()
#4 {main}
$ php --version
HipHop VM 4.0.0 (rel)
Compiler: 1549919142_639457389
Repo schema: 9466575221a6d6d3e932fd671f6a488a9119de5c
$ composer --version
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'array_pop() expects parameter 1 by reference, but the call was not annotated with '&'' in phar://composer.phar/bin/../src/../src/Composer/Util/Silencer.php:51
Stack trace:
#0 phar://composer.phar/bin/../src/../src/Composer/Util/Silencer.php(74): Composer\Util\Silencer::restore()
#1 phar://composer.phar/bin/../src/../src/Composer/Console/Application.php(71): Composer\Util\Silencer::call()
#2 phar://composer.phar/bin/composer(53): Composer\Console\Application->__construct()
#3 /home/travis/.phpenv/versions/hhvm-stable/bin/composer(25): include()
#4 {main}
0.87s$ composer install
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'array_pop() expects parameter 1 by reference, but the call was not annotated with '&'' in phar://composer.phar/bin/../src/../src/Composer/Util/Silencer.php:51
Stack trace:
#0 phar://composer.phar/bin/../src/../src/Composer/Util/Silencer.php(74): Composer\Util\Silencer::restore()
#1 phar://composer.phar/bin/../src/../src/Composer/Console/Application.php(71): Composer\Util\Silencer::call()
#2 phar://composer.phar/bin/composer(53): Composer\Console\Application->__construct()
#3 /home/travis/.phpenv/versions/hhvm-stable/bin/composer(25): include()
#4 {main}
The command "composer install" failed and exited with 255 during .
Your build has been stopped.
```
HHVM 4.0.0 was released today (11th Feb) https://hhvm.com/blog/2019/02/11/hhvm-4.0.0.html