The change is here: https://github.com/prooph/event-store/pull/210/commits/61465535e6acc657c80a1bab7ab909470cf579e8

Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

"Freezes"? Could it be an infinite loop somewhere? Use a debugger to find out exactly where and when the problem is.

I tried to create an isolated test script where you don't need to clone the event-store repository, and run the phpunit tests yourself. Unfortunately I was not successful so far. But I gave you the required links, so you actually are able to reproduce this issue, it's just a little more work than running a simple 20 line test-script.

PHP hangs forever exactly at this line:
https://github.com/prooph/event-store/pull/210/commits/61465535e6acc657c80a1bab7ab909470cf579e8#diff-cded286a30dea4e80fa00f01cfa1a8fbL73

It does not crash, so maybe it's an infinite loop, but not in the userland code, but in PHP itself.

Here is the shortest script to reproduce the issue.

<?php

$iterator1 = new \AppendIterator();
$iterator2 = new \AppendIterator();

$events = new \ArrayIterator(['event']);

$iterator1->append($events);
$iterator2->append($events);

// without this foreach, i will not hang
foreach ($iterator2 as $event) {
}

$events = new \ArrayIterator(['event']);
$iterator1->append($events);
$iterator2->append($events);

This bug is also there in PHP 5.6 and 7.0!
See also this test-script:

<?php

$iterator1 = new \AppendIterator();
$iterator2 = new \AppendIterator();

$events = new \ArrayIterator(['event']);

$iterator1->append($events);
$iterator2->append($events);

// without this foreach, i will not hang
foreach ($iterator2 as $event) {
}

$iterator1->append(new \ArrayIterator());

Narrowed it down to this while loop.

https://github.com/php/php-src/blob/PHP-7.1.0RC3/ext/spl/spl_iterators.c#L3379

Changed version, so far as I can see, this has been a bug since 5.2 ...