Event Loop is a concept from Javascript. In a single threaded execution, the code is waiting for the activity to get over before moving ahead. Event Loop helps us utilize the waiting period to execute code. PHP supports Event loop. There are multiple libraries which help us use event loop in our project https://github.com/reactphp https://github.com/icicleio https://github.com/asyncphp/doorman Don't be surprised if this question pops up as one of the top advanced PHP interview questions in your next attempt.