Description:
------------
I assume this is a feature: str_replace recursively replaces when both $search and $replace are arrays.

It's too easy for this to cause runaway memory consumption though. 

I've never needed to take advantage of the recursive nature of the function, and wouldn't expect many others to.

The code below is enough to hang my system. 

Reproduce code:
---------------
for ($i = 0; $i < 50; $i++) {
  $arr1[$i] = "foo";
  $arr2[$i] = "foobarfoo";
}

$str = "foobar";
echo str_replace($arr1, $arr2, $str);

Expected result:
----------------
Ideally:
foobarfoobar

Actual result:
--------------
foobarfoobarfoobarfoobarfoobarfoobarfoobarfoobar....