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What is a RESTful API, and how do standard HTTP methods like GET, POST, PUT, and DELETE map to common data operations? How does an API typically use HTTP status codes to communicate the result of a request, and what do common codes like 200, 404, and 500 represent? How does authentication typically work for a RESTful API, and what is the difference between session based authentication and token based authentication using something like a JSON Web Token? How should a well designed RESTful API structure its endpoints and URL naming conventions to remain intuitive and consistent as it grows? How should a RESTful API handle versioning as its design evolves over time, and what are the tradeoffs between common approaches like URL based versioning and header based versioning? How should a RESTful API be designed to handle complex error responses consistently, including validation errors affecting multiple fields at once?
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How does API rate limiting work, and why is it an important protection mechanism for a public facing RESTful API?

Intermediate

Answer

Rate limiting restricts how many requests a specific client, typically identified by an API key or IP address, can make within a given time window, protecting your API's backend resources from being overwhelmed by either legitimate heavy usage or a malicious attempt to abuse the API, and a well designed rate limited API typically communicates the current limit status back to the client through response headers, letting well behaved clients proactively adjust their request rate before actually hitting the limit and receiving a rejected request.
if ($requestCount > $rateLimit) {
    http_response_code(429);
    header('Retry-After: 60');
    echo json_encode(['error' => 'Rate limit exceeded']);
}
Real-world example A public API implements rate limiting that allows one hundred requests per minute per API key, returning a clear 429 status code along with a Retry-After header whenever a client exceeds that limit, letting well behaved client applications adjust their behavior accordingly.

Common follow-ups: What algorithms are commonly used to implement rate limiting, such as token bucket or sliding window?;How do you communicate remaining rate limit quota to clients through response headers?

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All 7 RESTful API Development with PHP questions