Return a response to the client before processing anything

respond-quickly.php

<?php

// Return our response as quickly as possible.
header('Connection: close');

ob_start();
header('HTTP/1.0 200 OK');
ob_end_flush();
flush();

// This immediately sends our response to the client; I hope you didn't have any other headers to set.

// Now, you can do all your other processing.

sample code to send a response as a api endpoint and then follow with a request in return in later code