Generating short keys from large ID numbers

How do the following sites make such short keys?

http://tinyurl.com/djf7u5
http://bit.ly/dcj7i
http://digg.com/d1oHPZ

I'll give anyone who can help a digital beer

 

just use md5() with long url as the function input to generate short key
store the short key with url in a table
when someone accesses short url check the short code in the table and find out the long url

 

http://us2.php.net/base_convert should do it, convert to basee 35

 

They are not hashes of the URL.

Those sites just use a sequential ID number for each URL they store, and encode that ID number to create the "shortened" URL. So instead of trying to hash a 50-character URL, they're just encode a 7-or-so-digit number.

 

<?php
$key_chars = array ("a","b",   "c",   "d",  "e",  "f",   "g",
     "h",   "i",   "j",  "k",
     "l",  "m", "n", "o", "p",  "q", 
    "r",  "s",  "t",  "u",  "v",
     "w",  "x",  "y", "z",
	"0","1", "2", "3", "4","5", "6", "7", "8", "9" ) ;
$length=6;
$text="";
for ($i = 0; $i < $length; $i++) {
    $text .= $key_chars[rand(0, count($key_chars)-1)];
}
echo $text;
?>

PHP:

 

How are you going to to decode that though? It's random, you're not actually encoding any useful information.