Description:
------------
In str_replace(), if you put as a replace string a call to a function (that returns a string), this call is executed always, even if the text to be replaced is not found in the text.

Reproduce code:
---------------
function some_func()
{
  echo "some_func() was called!\n";
  return 'bar';
}

echo str_replace('foo', some_func(), "aaaaa");

Expected result:
----------------
aaaa

Actual result:
--------------
some_func() was called!
aaaa