Simple longpoll functionality between JavaScript and PHP

longpoll.js

function longpoll(src, onload, onerror) {
	// create modified header
	var ifModifiedSince;

	// create request
	function onrequest() {
		var xhr  = new XMLHttpRequest();

		xhr.open('GET', src);

		// attach modified header
		xhr.setRequestHeader('If-Modified-Since', ifModifiedSince);

		// on request changes
		xhr.onreadystatechange = function () {
			// check if request finished
			if (xhr.readyState === 4 && xhr.status) {
				// if status returns error, goto error
				if (/^[45]/.test(xhr.status)) xhr.onerror();
				else {
					// conditionally run load callback
					if (typeof onload === 'function') onload.call(xhr, xhr.responseText);

					// update modified header
					ifModifiedSince = xhr.getResponseHeader('Last-Modified');

					// repeat request
					onrequest();
				}
			}
		};

		// on request errors
		xhr.onerror = function () {
			// conditionally run error callback
			if (typeof onerror === 'function') onerror.call(xhr, xhr.statusText);
		};

		// initiate request
		xhr.send(null);
	}

	// run request
	onrequest();
}

longpoll.php

<?php

$SRC     = 'test.json'; // path to file
$DELAY   = 20;  // measured in milliseconds (1000 = 1 second)
$TIMEOUT = INF; // measured in milliseconds, infinite will never timeout
$MIME    = 'text/plain; charset=utf-8'; // mime type and charset

// set indefinite execution time for this script
set_time_limit(0);

// get modified header as time
$IF_MODIFIED_SINCE = strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']);

while (true) {
	// clear file status cache (allow multiple checks on the same file)
	clearstatcache();

	// get file modified time
	$LAST_MODIFIED = filemtime($SRC);

	// if modified times do not match or timeout expires
	if ($LAST_MODIFIED !== $IF_MODIFIED_SINCE || !$TIMEOUT) {
		// set cache headers to not cache
		header('Cache-Control: private, max-age=0, no-cache');
		// set mime type
		header('Content-Type: '.$MIME);
		// set updated modified header
		header('Last-Modified: '.gmdate('D, d M Y H:i:s T', $LAST_MODIFIED));

		// print the response
		print(file_get_contents($SRC));

		// end
		break;
	}
	// otherwise
	else {
		// reduce timeout by delay
		$TIMEOUT -= $DELAY;

		// wait for delay
		usleep($DELAY * 1000);

		// repeat
		continue;
	}
}