Fiber::isRunning() will only return true if you call it from within a Fiber while it is running.<?php$fiber = new Fiber(function ($fiber) { echo "Inside Fiber. isRunning: "; var_dump($fiber->isRunning()); // Here is true Fiber::suspend(); echo "Inside Fiber again after resume(). isRunning: "; var_dump($fiber->isRunning()); // true again});echo "Before Fiber launch. isRunning: ";var_dump($fiber->isRunning()); // Here is false$fiber->start($fiber);echo "After suspend. isRunning: ";var_dump($fiber->isRunning()); // Here is false$fiber->resume();echo "After resume. isRunning: ";var_dump($fiber->isRunning()); // Here is false?>Result:Before Fiber launch. isRunning: bool(false)Inside Fiber. isRunning: bool(true)After suspend. isRunning: bool(false)Inside Fiber again after resume(). isRunning: bool(true)After resume. isRunning: bool(false)