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.
Nota: The behaviour of this function was buggy until PHP 4.3.0. Before this, the str was also modified, as if passed by reference.