Question

Is it possible to encode user’s ID# to text value?

For example if the user ID# is ‘50624’ is it possible to convert it to 7 characters text value, something like ‘DEGXCVG’?

If I use MD5 for this function I get 16-byte hash value.
The result value should not be longer than 7 or 8 characters.

Edit:
I want that each user has a different text code obtained from the id number.

Answer 1

If a hash function such as MD5 can be used instead of encryption, you could try crc32() instead, which produces shorter (but with much higher chances of collisions) strings.

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If you just care about encoding, you could do something simpler:

function encode($id) {
  $id_str = (string) $id;
  $offset = rand(0, 9);
  $encoded = chr(79 + $offset);
  for ($i = 0, $len = strlen($id_str); $i < $len; ++$i) {
    $encoded .= chr(65 + $id_str[$i] + $offset);
  }
  return $encoded;
}

function decode($encoded) {
  $offset = ord($encoded[0]) - 79;
  $encoded = substr($encoded, 1);
  for ($i = 0, $len = strlen($encoded); $i < $len; ++$i) {
    $encoded[$i] = ord($encoded[$i]) - $offset - 65;
  }
  return (int) $encoded;
}

Example:

var_dump(encode(50624)); // string(6) "TKFLHJ"

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Encryption version:

define('CRYPTO_KEY', 'Secret key');

function encrypt($id) {
  return base64_encode(mcrypt_encrypt(MCRYPT_BLOWFISH, CRYPTO_KEY, (string) $id, MCRYPT_MODE_ECB));
}

function decrypt($encrypted) {
  return (int) base64_decode(mcrypt_decrypt(MCRYPT_BLOWFISH, CRYPTO_KEY, $encrypted, MCRYPT_MODE_ECB));
}