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Asked: 2026-05-26T03:58:56+00:00

I am trying to better understand how a function works. I am not familiar

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I am trying to better understand how a function works. I am not familiar with how the function below works and I was looking for some help and possibly an explanation. I get most of it except one part that uses math functions like floor(). The part that confuses me the most is the do…while loop. How exactly does using $id, floor() and $base convert a long url into a short url? Also, what exactly is it iterating through where you need a do…while loop to do it?

$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
$base  = strlen($chars);     // base 62
$site  = 'http://short.co';  // Replace with your domain

// ...Connect to MySQL server here...

function shorten_url($url)
{
  global $chars, $site, $base;

  $hash  = md5($url); 
  $alnum = NULL; //

  // Check if URL is already exist on db
  // Otherwise add this URL to table
  $res = mysql_query("SELECT id FROM urls WHERE url_hash='$hash'");

  if (mysql_num_rows($res)) {
    $row = mysql_fetch_object($res);
    $id = $row->id;
  } else {
    mysql_query("INSERT INTO urls (url, url_hash) VALUES ('$url', '$hash')");
    $id = mysql_insert_id();
  }

  // Convert id to base 62 and decode to alphanumeric
  do {
    $alnum = $chars[($id%$base)].$alnum;
  } while ($id = floor($id/$base));

  return "$site/$alnum";
}

If someone could provide an explanation without a ton of math jargon…I would appreciate it. Thanks.

I got the function from: http://bsd-noobz.com/blog/how-to-create-url-shortening-service-using-simple-php

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1 Answer

  1. Editorial Team

    You are probably only used to thinking of numbers that have 0 through 9 in them. What would have happened if we had 12 fingers instead of 10? Maybe we would have made another digit beyond 9?

    In your code, there are 62 different “digits” in your number system. You start counting with “a”, then “b”, then “c”, and so forth. When you run out of them, you start using two digits – “aa”, then “ab” until you run out those… then add another digit.

    When you create a new shortened URL, you insert it in a database and it gets an “id” number. The function converts it from the 10 digit numbers (called “base 10”) to the 64 digit numbers.

    When you want to access a URL, you convert back to base 10, look it up in the database, and serve it back to the user.

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