This page contains a ROT13 Encoder/Decoder, sometimes called ROT13 encryption. Below the nifty form is an explanation of what ROT13 is and how it works.
Encoding is not Encryption!!
ROT13 simply takes the alphabetical characters of the input and will ROTate them 13 places. The 13 places depends on the position of the letter, lower letters get pushed up, and the higher letters get pulled down. In the below example the letters are shown with their position, and the ASCII decimal value;
Character Position ASCII Shift Result A 1 65 +13 N B 2 66 +13 O M 13 77 +13 Z N 14 78 -13 A Y 25 89 -13 L Z 26 90 -13 M
So as you can see the ROT13 is not a very secure encoding scheme, the encoding is really just a simple character subsistution. Sometimes the ROT13 is called the Caesar-cypher as it is said he used ROT13 for communication during the Pelloponesian Wars
If you are using PHP greater than version 4.2.0 you can use their function str_rot13, otherwise you'll have to code it yourself.