Description:
------------
函数原型为：
string str_replace(mixed $search, mixed $replace, mixed $subject [, int &$count])

需要跟踪的参数为$subject 和 $replace 而不是$search 和 $replace

Test script:
---------------

<?php 
$a = "tainted string" . ".";
taint($a); //must use concat to make the string not a internal string(introduced in 5.4)

$b = str_replace("str", "btr"， $a);
var_dump(is_tainted($b));

?>




Expected result:
----------------
bool(true)

Actual result:
--------------
bool(false)