Anything in your error log? You could be hitting a configured memory limit.

Nothing at all. I don't get any errors on screen either.

To be clear: you're saying the page *hangs*, meaning it never completes? Or it completes but leaves an empty page? And is that number near 2 million or 2 *billion*?

What about

<?php

error_reporting(-1);
ini_set('display_errors', true);

echo "start<br>\n"; flush();

$data = '';
for($i = 0; $i < 3000000; $i++) {
	$data .= 1;
}

echo "end<br>\n"; flush();
echo $data; flush();

?>

2 million. And the page hangs, never completes. Bizarrely if I view the same page, using the same wifi connection on a mobile it loads in under a second.

This doesn't seem like a PHP problem then. PHP has no idea, and doesn't in any way care what kind of client is used for the request. If one works and another doesn't, the problem is elsewhere.

I'm not sure. I can do pretty much anything with the string except echo it. I can write it to a file for instance, but I can't use readfile() and output it on screen, it's really weird. Although I can do all this on my local machine with no errors.

I've read elsewhere that it may have something to do with Nagles Algorithm and tcp/ip buffering, but that's a bit over my head.

I've been through all the settings on the server that I can think of but nothing is fixing it.

Right, but you are getting into technical support questions here and this is a bug tracker. PHP doesn't do the actual network communications. Your web server and operating system does that. There is nothing here that indicates a PHP bug of any kind.