Modern web APIs spend much of their time waiting for I/O, such as database queries, HTTP calls, or external services, rather than executing PHP code. PHP 8 introduced Fibers as a low-level concurrency primitive. By PHP 8.3, they are stable enough for production use. Fibers do not make PHP multithreaded, but they do allow multiple I/O-bound operations to make progress without blocking each other. In this article, we will examine how Fibers work, where they make sense, and how to use them to reduce request latency in real-world APIs using clear, practical examples. by Sarah Aburu