extractCssFiles.php

<?php

function extractCssFiles(array $manifest, string $manifestKey): array
{
    $entry = $manifest[$manifestKey] ?? null;

    if (!$entry) {
        return [];
    }

    $cssFiles = array_values($entry['css'] ?? []);
    $imports = array_merge($entry['imports'] ?? [], $entry['dynamicImports'] ?? []);
    
    $importFiles = array_map(function ($manifestKey) use ($manifest, $manifestKey) {
        return $this->extractCssFiles($manifest, $manifestKey);
    }, $imports);

    array_push($cssFiles, ...$importFiles);

    return array_unique($cssFiles);
}

extractCssFilesReduce.php

<?php

function extractCssFiles(array $manifest, string $manifestKey): array
{
    $entry = $manifest[$manifestKey] ?? null;

    if (!$entry) {
        return [];
    }

    $cssFiles = array_values($entry['css'] ?? []);
    $imports = array_merge($entry['imports'] ?? [], $entry['dynamicImports'] ?? []);

    return array_reduce($imports, function ($cssFiles, $manifestKey) use ($manifest) {
        return array_unique(array_merge($cssFiles, $this->extractCssFiles($manifest, $manifestKey)));
    }, $cssFiles);
}