Description:
------------
When creating a large amount of objects without storing them, the memory usage keeps increasing, but only when the script is executed as an HTTP request.
When executed over the CLI, the behaviour is as expected: the memory usage is stable then.

The issue can be reproduced using the php:7.2.x-apache-stretch Docker images and the script below.
The problem does not occur everytime you run the script: most of the times, the first request after you boot the container will not show the problem, but each following request does.

The problem is introduced in PHP 7.2: earlier versions of PHP do not show this problem. 
Creating arrays or scalars instead of objects works as it should and does not retain memory.
Wrapping the loop in a function, to trigger a cleanup after the function has executed does not help.

Test script:
---------------
<?php
$memory = memory_get_usage();
printf("before iterating, memory %d\n", $memory);

for($i = 0; $i < 1000000; $i++) {
	if (($new_memory = memory_get_usage()) > $memory) {
		$memory = $new_memory;
		printf("iteration %d, memory %d\n", $i, $memory);
	}
	new \stdClass();
}
printf("after iterating, memory %d\n", memory_get_usage());

Expected result:
----------------
before iterating, memory 385728
iteration 0, memory 385760
after iterating, memory 385760

(obtained by running the script over the CLI in the Docker container)

Actual result:
--------------
before iterating, memory 378928
iteration 0, memory 379040
iteration 1024, memory 387232
iteration 2048, memory 403616
iteration 4096, memory 436384
iteration 8192, memory 501920
iteration 16384, memory 632992
iteration 32768, memory 895136
iteration 65536, memory 1419424
iteration 131072, memory 2468024
iteration 262144, memory 4565176
iteration 524288, memory 8759480
after iterating, memory 8759480


As you can see, the script goes from around 370 KB to using over 8 MB.

(obtained by running the script over HTTP)