This function performs the ROT13 encoding on the
str argument and returns the resulting
string. The ROT13 encoding simply shifts every letter by 13
places in the alphabet while leaving non-alpha characters
untouched. Encoding and decoding are done by the same function,
passing an encoded string as argument will return the original version.
Note: The behaviour of this function was buggy until PHP 4.3.0. Before this, the
strwas also modified, as if passed by reference.