With Twitter and its 140 characters limit leading all the trends, URL shortening services are in extremely high demand. We have seen TinyURL in the past, now Bit.ly, Is.gd, Tr.im and dozens of others are joining the party to share the cake. Honestly saying, to me they’re all the same (except for ➡.ws for its cool name). With that being said however, it is interesting how they are doing it out there - what mechanism / algorithm / buzzword-here?

I curiously did some “research”es (well, another buzzword), and it seems in order to create the shorten URLs they are following the same steps:

1. Insert the original (long) URL into the database
2. Get the insert ID. If the URL already exists, take its row ID. Let’s say we got 123456.
3. Convert the ID 123456 into something even shorter, let’s say “am4k”
4. Make use of Apache’s mod_rewrite so that any request to http://host.com/am4k will reach http://host.com/redir_script.php?code=am4k instead.
5. In redir_script.php the value “am4k” is converted back into base10′s 123456 and the corresponding URL is queried back from database
6. If a URL is found, redirect the request to it.

It is simple enough… except for step 3 and 4. Which conversion is there, and how is the .htaccess written?

So I did some other buzzword-here, and it turned out base36 encoding is taking place. According to Wikipedia, “the choice of 36 is convenient in that the digits can be represented using the Arabic numerals 0-9 and the Latin letters A-Z”. There is a handy conversion table too:

Even better, PHP takes only one line of code to convert between decimal and base36, as shown on the same Wiki page:

$base_36 = "ZAQFG"; //Sample Base 36 Number
$decimal = "7654321"; //Sample Decimal Number
echo base_convert($base_36,36,10); //Outputs $base_36 converted to decimal
echo base_convert($decimal,10,36); //Outputs $decimal converted to base 36

In our example, 123456 will be converted into its tidy equivalent base36 2n9c (not am4k, my very bad).

As for the mod_rewrite thing, as I am not a .htaccess master, I copied this thing that has been working perfectly so far from CodeIgniter forum:

RewriteEngine On
RewriteBase /
#Checks to see if the user is attempting to access a valid file,
#such as an image or css document, if this isn't true it sends the
#request to redir.php.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ redir.php?code=$1 [L]

With this, all of the hardest parts are done, leave the remains being simple MySQL database, queries and HTML form handling. To save you from getting boring, I’ll omit it.

Instead, let’s see a demo hereThe source code of the demo are can be downloaded from the same server