Description:
------------
If 'search' and 'subject' has equal length and both begins with equal substring with chr(0) inside - str_replace() considers that 'search' and 'subject' equals.

Reproduce code:
---------------
<?php
$search =  "qx\0qqqqqqqqqq";
$subject = "qx\0xxxxxxxxxx";
$replace = "any text";

$result = str_replace ( $search, $replace, $subject );

echo $search . "\n" . $replace . "\n" . $subject . "\n" . $result . "\n";
?>

Expected result:
----------------
qx qqqqqqqqqq
any text
qx xxxxxxxxxx
qx xxxxxxxxxx

Actual result:
--------------
qx qqqqqqqqqq
any text
qx xxxxxxxxxx
any text