Handling downloads safely in PHP requires careful coordination to prevent race conditions, ensure atomic writes, and maintain integrity. Use flock or a distributed lock library appropriate to your deployment, combine locks with temp-file + rename patterns, validate content, and avoid long blocking in request handlers by moving heavy work to background jobs when needed. Symfony Lock and Redis-backed patterns are common, and standard PHP functions (flock, file_put_contents, rename, cURL/Guzzle) provide the building blocks.
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