I figured out why I was having issues with the Composer autoloader in D12874. In the [[https://github.com/nikic/PHP-Parser/ | PHP-Parser]] library, there exists the following code:
```lang=php
<?php
if (!class_exists('PhpParser\Autoloader')) {
require __DIR__ . '/PhpParser/Autoloader.php';
}
PhpParser\Autoloader::register();
```
It seems that the issue is that `__phutil_autoload` will throw an `Exception` instead of allowing `class_exists` to return `false`. For a minimal example, consider the following code:
```lang=php
#!/usr/bin/env php
<?php
require_once dirname(__FILE__).'/__init_script__.php';
if (class_exists('FooBar')) {
echo "Success!\n";
}
```