Implement ROT13 with PHP - php

Question

I found a string after reading funny things about Jon Skeet, and I guessed that it was in ROT13. Before just checking my guess, I thought I'd try and decrypt it with PHP. Here's what I had:
$string = "Vs lbh nfxrq Oehpr Fpuarvre gb qrpelcg guvf, ur'q pehfu lbhe fxhyy jvgu uvf ynhtu.";
$tokens = str_split($string);
for ($i = 1; $i <= sizeof($tokens); $i++) {
$char = $tokens[$i-1];
for ($c = 1; $c <= 13; $c++) {
$char++;
}
echo $char;
}
My string comes back as AIaf you aasakaead ABruacae Sacahnaeaiaer to adaeacrypt tahais, ahae'ad acrusah your sakualal waitah ahais alaauagah.
My logic seems quite close, but it's obviously wrong. Can you help me with it?

Answer 1

Try str_rot13.
http://us.php.net/manual/en/function.str-rot13.php
No need to make your own, it's built-in.

Answer 2

Here is a working implementation, without using the nested loop. You also don't need to split the string into an array, since you can index individual characters just like an array with strings in PHP.
You need to know that ASCII upper-case characters range from 65 - 99, and lower-case characters range from 97 - 122. If the current character is in one of those ranges, add 13 to its ASCII value. Then, you check if you should have rolled over to the beginning of the alphabet. If you should've rolled over, subtract 26.
$string = "Vs lbh nfxrq Oehpr Fpuarvre gb qrpelcg guvf, ur'q pehfu lbhe fxhyy jvgu uvf ynhtu.";
for ($i = 0, $j = strlen( $string); $i < $j; $i++)
{
// Get the ASCII character for the current character
$char = ord( $string[$i]);
// If that character is in the range A-Z or a-z, add 13 to its ASCII value
if( ($char >= 65 && $char <= 90) || ($char >= 97 && $char <= 122))
{
$char += 13;
// If we should have wrapped around the alphabet, subtract 26
if( $char > 122 || ( $char > 90 && ord( $string[$i]) <= 90))
{
$char -= 26;
}
}
echo chr( $char);
}
This produces:
If you asked Bruce Schneier to decrypt this, he'd crush your skull with his laugh.

Answer 3

If you want to do this yourself, instead of using an existing solution, you need to check whether each letter is at the first or second half of the alphabet. You can't naively add 13 (also, why are you using a loop to add 13?!) to each character. You must add 13 to A-M and subtract 13 from N-Z. You must also, not change any other character, like space.
Alter your code to check each character for what it is before you alter it, so you know whether and how to alter it.

Answer 4

This is not working because z++ is aa
$letter = "z";
$letter++;
echo($letter);
returns aa not a
EDIT: A possible alternative solution not using the built in is
$string = "Vs lbh nfxrq Oehpr Fpuarvre gb qrpelcg guvf, ur'q pehfu lbhe fxhyy jvgu uvf ynhtu.";
$tokens = str_split($string);
foreach($tokens as $char)
{
$ord = ord($char);
if (($ord >=65 && $ord <=90 ) || ($ord >= 97 && $ord <= 122))
$ord = $ord+13;
if (($ord > 90 && $ord < 110) || $ord > 122)
$ord = $ord - 26;
echo (chr($ord));
}

Answer 5

Just a few years late to the party, but I thought I'd give you another option to do this
function rot13($string) {
// split into array of ASCII values
$string = array_map('ord', str_split($string));
foreach ($string as $index => $char) {
if (ctype_lower($char)) {
// for lowercase subtract 97 to get character pos in alphabet
$dec = ord('a');
} elseif (ctype_upper($char)) {
// for uppercase subtract 65 to get character pos in alphabet
$dec = ord('A');
} else {
// preserve non-alphabetic chars
$string[$index] = $char;
continue;
}
// add 13 (mod 26) to the character
$string[$index] = (($char - $dec + 13) % 26) + $dec;
}
// convert back to characters and glue back together
return implode(array_map('chr', $string));
}