a) str_rot13() changes it's given variable.
b) str_rot13() changes OTHER variables.

8<----------------------------
$ cat test.php
<?php
 $first = "boo";
 $second = $first;
 $rot = "";

 echo "1: ".$first."\n";
 echo "2: ".$second."\n";
 echo "3: ".$rot."\n";

 $rot = str_rot13($second);

 echo "1: ".$first."\n";
 echo "2: ".$second."\n";
 echo "3: ".$rot."\n";
?>
--------------------------->8

8<-----------------------
$ php4 -q test.php
1: boo
2: boo
3:
1: obb
2: obb
3: obb
----------------------->8

The changing of 1 is quite odd (subbug B) and very annoying.
The changing of 2 is probably because it happens in place.

Either way $first shouldn't be touched at all.

This is on Debian Linux/unstable, but others are probably affected too.