Composer 自动加载的 PHP 版本问题疑惑？

Composer在初始化的时候，分为两种初始化方式：php版本为5.6以下的和php版本5.6以上的：

$useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION');
if ($useStaticLoader) {
   require_once __DIR__ . '/autoload_static.php';
   call_user_func(\Composer\Autoload\ComposerStaticInit
   832ea71bfb9a4128da8660baedaac82e::getInitializer($loader));
} else {
    $map = require __DIR__ . '/autoload_namespaces.php';
    foreach ($map as $namespace => $path) {
       $loader->set($namespace, $path);
    }
    $map = require __DIR__ . '/autoload_psr4.php';
    foreach ($map as $namespace => $path) {
       $loader->setPsr4($namespace, $path);
    }
    $classMap = require __DIR__ . '/autoload_classmap.php';
    if ($classMap) {
        $loader->addClassMap($classMap);
    }
  }

getInitializer函数：

     public static function getInitializer(ClassLoader $loader)
  {
    return \Closure::bind(function () use ($loader) {
        $loader->prefixLengthsPsr4 = ComposerStaticInit832ea71bfb9a4128da8660baedaac82e::$prefixLengthsPsr4;
        $loader->prefixDirsPsr4 = ComposerStaticInit832ea71bfb9a4128da8660baedaac82e::$prefixDirsPsr4;
        $loader->prefixesPsr0 = ComposerStaticInit832ea71bfb9a4128da8660baedaac82e::$prefixesPsr0;
        $loader->classMap = ComposerStaticInit832ea71bfb9a4128da8660baedaac82e::$classMap;
    }, null, ClassLoader::class);

其中classMap：

public static $classMap = array (
    'App\\Console\\Kernel' => __DIR__ . '/../..' . '/app/Console/Kernel.php',
    'App\\Exceptions\\Handler' => __DIR__ . '/../..' . '/app/Exceptions/Handler.php',
    'App\\Http\\Controllers\\Auth\\ForgotPasswordController' => __DIR__ . '/../..' . '/app/Http/Controllers/Auth/ForgotPasswordController.php',
    'App\\Http\\Controllers\\Auth\\LoginController' => __DIR__ . '/../..' . '/app/Http/Controllers/Auth/LoginController.php',
    'App\\Http\\Controllers\\Auth\\RegisterController' => __DIR__ . '/../..' . '/app/Http/Controllers/Auth/RegisterController.php',
    ...)

请问各位大牛，这段代码为神马要限制php版本5.6呢？我各处都没有找到原因，看了php的版本区别也没用头绪，请大牛解惑