Description:
------------
inspired by bug #44069 (Huge memory usage with concatenation using . instead of .=)
string concatenation with something like this is really slow:
$newstring = ($newstring . $string); // slow nearly 6 seconds on my machine



Reproduce code:
---------------
$start_time = microtime(true);

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

echo "Length: " .strlen($string)."\n";
echo "Memory Before:\n".memory_get_usage(true)."\n";
$newstring = "";

for($i = 1; $i <= 2000; $i++)
{
	// $newstring .= $string; // fast 0.02 seconds
	$newstring = ($newstring . $string); // slow nearly 6 seconds on my machine
}
$end_time = microtime(true);
echo "start_time: $start_time\n";
echo "end_time: $end_time\n";

echo "Memory After:\n".memory_get_usage(true)."\n";
echo "Total Length of String:\n".strlen($newstring)."\n";
echo "\n=====\n";
echo "seconds: " . ($end_time-$start_time) . "\n";
echo "\n";

Expected result:
----------------
Length: 1050
Memory Before:
524288
start_time: 1222714498.4688
end_time: 1222714498.4977
Memory After:
2883584
Total Length of String:
2100000

=====
seconds: 0.028898954391479

Actual result:
--------------
Length: 1050
Memory Before:
524288
start_time: 1222714527.1094
end_time: 1222714532.8964
Memory After:
2883584
Total Length of String:
2100000

=====
seconds: 5.7869839668274