Core

the PackageManagerTest::getPackageReturnsTheSpecifiedPackage is failing on hhvm, because it HHVM doesn't support calling key() on objects.
So it is failing now with "Fatal error: Cannot access empty property in..."
Responsible code is in Package->getNamespace()

$namespaces = $manifest->autoload->{'psr-0'};
if (count($namespaces) === 1) {
    $namespace = key($namespaces);
}

it can be solved by first cast object to an array

$namespaces = (array)$manifest->autoload->{'psr-0'};
if (count($namespaces) === 1) {
    $namespace = key($namespaces);
}

quote from
https://github.com/facebook/hhvm/blob/master/hphp/doc/inconsistencies#L90-95
about this inconsistency:

(5) Object internal cursors

Under Zend PHP, objects have an internal cursor (similar to the array internal
cursor) that can be used to iterate over the object's properties. Under HipHop
VM, objects do not have internal cursors, and the next(), prev(), current(),
key(), reset(), end(), and each() builtin functions do not support objects.