Здравствуйте, Сергей Науменко.

Вот например шифрование алгоритмом xxtea.

У нас есть в распоряжении две функции: xxtea_encrypt и xxtea_decrypt. Остальные функции носят вспомогательный характер.
require_once 'xxtea.inc.php';

$cipher = xxtea_encrypt('My test string', 'My test password');
echo xxtea_decrypt($cipher, 'My test password');

Приложение:
<?php
/* XXTEA encryption arithmetic library.
*
* Copyright (C) 2006 Ma Bingyao <andot@ujn.edu.cn>
* Version: 1.5
* LastModified: Dec 5, 2006
* This library is free. You can redistribute it and/or modify it.
*/

function long2str($v, $w) {
$len = count($v);
$n = ($len - 1) << 2;
if ($w) {
$m = $v[$len - 1];
if (($m < $n - 3) || ($m > $n)) return false;
$n = $m;
}
$s = array();
for ($i = 0; $i < $len; $i++) {
$s[$i] = pack("V", $v[$i]);
}
if ($w) {
return substr(join('', $s), 0, $n);
} else {
return join('', $s);
}
}

function str2long($s, $w) {
$v = unpack("V*", $s. str_repeat("\0", (4 - strlen($s) % 4) & 3));
$v = array_values($v);
if ($w) {
$v[count($v)] = strlen($s);
}
return $v;
}

function int32($n) {
while ($n >= 2147483648) $n -= 4294967296;
while ($n <= -2147483649) $n += 4294967296;
return (int)$n;
}

function xxtea_encrypt($str, $key) {
if ($str == "") {
return "";
}
$v = str2long($str, true);
$k = str2long($key, false);
if (count($k) < 4) {
for ($i = count($k); $i < 4; $i++) {
$k[$i] = 0;
}
}
$n = count($v) - 1;

$z = $v[$n];
$y = $v[0];
$delta = 0x9E3779B9;
$q = floor(6 + 52 / ($n + 1));
$sum = 0;
while (0 < $q--) {
$sum = int32($sum + $delta);
$e = $sum >> 2 & 3;
for ($p = 0; $p < $n; $p++) {
$y = $v[$p + 1];
$mx = int32((($z >> 5 & 0x07ffffff) ^ $y << 2) + (($y >> 3 & 0x1fffffff) ^ $z << 4)) ^ int32(($sum ^ $y) + ($k[$p & 3 ^ $e] ^ $z));
$z = $v[$p] = int32($v[$p] + $mx);
}
$y = $v[0];
$mx = int32((($z >> 5 & 0x07ffffff) ^ $y << 2) + (($y >> 3 & 0x1fffffff) ^ $z << 4)) ^ int32(($sum ^ $y) + ($k[$p & 3 ^ $e] ^ $z));
$z = $v[$n] = int32($v[$n] + $mx);
}
return long2str($v, false);
}

function xxtea_decrypt($str, $key) {
if ($str == "") {
return "";
}
$v = str2long($str, false);
$k = str2long($key, false);
if (count($k) < 4) {
for ($i = count($k); $i < 4; $i++) {
$k[$i] = 0;
}
}
$n = count($v) - 1;

$z = $v[$n];
$y = $v[0];
$delta = 0x9E3779B9;
$q = floor(6 + 52 / ($n + 1));
$sum = int32($q * $delta);
while ($sum != 0) {
$e = $sum >> 2 & 3;
for ($p = $n; $p > 0; $p--) {
$z = $v[$p - 1];
$mx = int32((($z >> 5 & 0x07ffffff) ^ $y << 2) + (($y >> 3 & 0x1fffffff) ^ $z << 4)) ^ int32(($sum ^ $y) + ($k[$p & 3 ^ $e] ^ $z));
$y = $v[$p] = int32($v[$p] - $mx);
}
$z = $v[$n];
$mx = int32((($z >> 5 & 0x07ffffff) ^ $y << 2) + (($y >> 3 & 0x1fffffff) ^ $z << 4)) ^ int32(($sum ^ $y) + ($k[$p & 3 ^ $e] ^ $z));
$y = $v[0] = int32($v[0] - $mx);
$sum = int32($sum - $delta);
}
return long2str($v, true);
}
?>