http://rduarte.posterous.com/urls-curtas-e-seguras-usando-base62-base64-e

base62_functions.php

function base62_encode($num){
    $chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $base = 62;
    $result = '';
 
    while($num >= $base) {
        $r = $num%$base;
        $result = $chars[$r].$result;
        $num = $num/$base;
    }
    return $chars[$num].$result;
}
 
function base62_decode($id){
    $chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $base = 62;
    for ($i=0; $i<strlen($id); $i++) {
        $value = strpos($chars, $id[$i]);
        $num = $value * pow($base, strlen($id)-($i+1));
        $final += $num;
    }
    return $final;
}

enc_dec.php

function codificar($valor, $chave){
 
  if (($chave = codificacao_chave($chave)) !== false){
    list($inicio, $fim) = $chave;
  } else {
    return false;
  }
  $base = base64_encode($valor);
  $base = str_replace('=', '', $base);
  $base = strrev($base);
  $md5 = md5($valor);
  $base = (substr($md5, 0, $inicio) . $base . substr($md5, -$fim));
 
  return $base;
}
 
function decodificar($valor, $chave){
 
  if (($chave = codificacao_chave($chave)) !== false){
    list($inicio, $fim) = $chave;
  } else {
    return false;
  }
  $hash = (substr($valor, 0, $inicio) . substr($valor, -$fim));
  $base = substr($valor, 0, -$fim);
  $base = substr($base, $inicio);
  $base = strrev($base);
  $cmp = (strlen($base) % 4);
  if ($cmp > 0) $base .= str_repeat('=', $cmp);
  $base = base64_decode($base);
  $md5 = md5($base);
  $md5 = (substr($md5, 0, $inicio) . substr($md5, -$fim));
 
  return (($md5 == $hash) ? $base : false);
}
 
function codificacao_chave($chave){
 
  $valor = md5($chave);
  $t = strlen($valor);
  $dig1 = '';
  $dig2 = '';
 
  for ($i = 0; $i < $t; $i++){
    $ch = substr($valor, $i, 1);
    if (is_numeric($ch) && ($ch !== '0')){
      if ($dig1 == ''){
        $dig1 = $ch;
      } elseif ($dig2 == ''){
        $dig2 = $ch;
      } else {
        break;
      }
    }
  }
 
  return (($dig1 != '') && ($dig2 != '')) ? array($dig1, $dig2) : false;
}

test_base62.txt

8 equivale 8
9 equivale 9
10 equivale a
11 equivale b
12 equivale c
(...)
1033 equivale gF
1034 equivale gG
1035 equivale gH
1036 equivale gI

test_enc_dec.txt

100000 equivale 1wADMwATM6c65b (usando a chave 'chave-secreta')
200000 equivale 0wADMwAjM3f793 (usando a chave 'chave-secreta')
(...)

test_final.txt

ID:1000    http://url.com/u/1wADMwATM6c65b
ID:2000    http://url.com/u/0wADMwAjM3f793