Description:
------------
In a for-loop I concatenate two strings. If I at the same time fill a relatively small array, I get a huge memory consumption. This only happens if I concatenate the two strings using '.'. If I use '.=' the memory usage is as expected.

Reproduce code:
---------------
<?php

$string = str_repeat('This is a teststring.', 50);

echo 'Length: '.strlen($string).'<br>';
echo 'Memory Before:'.memory_get_usage(true).'<br>';

for($i = 1; $i <= 2000; $i++)
{
//	$newstring .= $string; //This uses an expected amount of mem.
	$newstring = $newstring . $string; //This uses very much mem.

	for($j = 1; $j <= 10; $j++)
	{
		$array[] = 'test';
	}
}
echo 'Memory After:'.memory_get_usage(true).'<br>';
echo 'Total Length of String: '.strlen($newstring).'<br>';

?>

Expected result:
----------------
Length: 1050
Before:262144
After:4456448
Length: 2100000


Actual result:
--------------
Length: 1050
Before:262144
After:161742848
Length: 2100000