composer中 自动加载函数中 getloader中为何要以php5.6作为区分$useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION');，这个区分主要区分的什么?

public static function getLoader()
    {
        if (null !== self::$loader) {
            return self::$loader;
        }

        spl_autoload_register(array('ComposerAutoloaderInit812e73c32a9b50cd0733a00f38392e8a', 'loadClassLoader'), true, true);
        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
        spl_autoload_unregister(array('ComposerAutoloaderInit812e73c32a9b50cd0733a00f38392e8a', 'loadClassLoader'));

        $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION');
        if ($useStaticLoader) {
            require_once __DIR__ . '/autoload_static.php';

            call_user_func(\Composer\Autoload\ComposerStaticInit812e73c32a9b50cd0733a00f38392e8a::getInitializer($loader));
        } else {
            $classMap = require __DIR__ . '/autoload_classmap.php';
            if ($classMap) {
                $loader->addClassMap($classMap);
            }
        }

        $loader->setClassMapAuthoritative(true);
        $loader->register(true);

        if ($useStaticLoader) {
            $includeFiles = Composer\Autoload\ComposerStaticInit812e73c32a9b50cd0733a00f38392e8a::$files;
        } else {
            $includeFiles = require __DIR__ . '/autoload_files.php';
        }
        foreach ($includeFiles as $fileIdentifier => $file) {
            composerRequire812e73c32a9b50cd0733a00f38392e8a($fileIdentifier, $file);
        }

        return $loader;
    }
}