Dump paths defined in composer.json into lib/paths.php to load in the kernel

PathDumper.php

<?php

use Composer\Script\Event;

class PathDumper
{
    static $defaults = [
        'symfony-app-dir'    => 'boot',
        'symfony-bin-dir'    => 'bin',
        'symfony-config-dir' => 'etc',
        'symfony-test-dir'   => 'tests',
        'symfony-var-dir'    => 'var',
        'symfony-web-dir'    => 'public',
    ];

    public static function dumpKernelPaths(Event $event)
    {
        $composer = $event->getComposer();
        $rootPackage = $composer->getPackage();

        $extra = self::addDefaultValues($rootPackage->getExtra());
        $rootPackage->setExtra($extra);

        $projectDir = getcwd();
        $paths = self::extractPaths($extra, $projectDir);

        $config = $composer->getConfig();
        $vendorDir = $config->get('vendor-dir');
        self::updatePaths($vendorDir, $paths);
    }

    private static function extractPaths(array $extra, $basePath)
    {
        $paths = [];
        foreach ($extra as $key => $value) {
            if (preg_match('~^symfony-([^-]+)-dir$~', $key, $matches)) {
                $paths[$matches[1]] = $basePath . '/' . trim($value, '/');
            }
        }

        return $paths;
    }

    private static function addDefaultValues(array $extra)
    {
        return array_merge(self::$defaults, $extra);
    }

    private static function updatePaths($directory, array $paths)
    {
        $dump = var_export($paths, true);

        file_put_contents(
            "{$directory}/paths.php",
            "<?php return {$dump};"
        );
    }
}