Determine best cdn for image and remember it - php

Question

I want a way to take any input (url) and get back a number between 1-4, distributed as even as possible 25% for any input. It's important that it gets the same value of 1-4 every time.
The reason I want this is so that I can create seemingly random and evenly disturbed content for a set of CNAMEs (subdomains) for a CDN. It would take pictures that were originally www.website.com/picture.png and output them as
cdn1.website.com/picture.png or
cdn2.website.com/picture.png or
cdn3.website.com/picture.png or
cdn4.website.com/picture.png
Effectively allowing me to bypass the browser restrictions set to a subdomain, giving me more parallel connections (Read more: http://yuiblog.com/blog/2007/04/11/performance-research-part-4/). The reason why I want the URL to always pass back to a specific CDN is for caching purposes; If the www.website.com/picture.png is first displayed as cdn1.website.com/picture.png and a second time around as cdn2.website.com/picture.png then the browser would not know that it has the same picture cached already under cdn1 and would download the same picture twice, rather than relying on cache.
Here the suggested php at it, but I as you can see from results that I don't get that 25% ratio I would like for small sample set. I am looking for alternatives that would also be somewhat close to 25% distribution for small samples.
<?php
$num_array = array();
for ($i = 1; $i <= 10000; $i++) {
$num_array[]=(crc32(genRandomURL()) % 4)+1;
}
print "<pre>";
print_r(array_count_values($num_array));
print "</pre>";
$num_array = array();
for ($i = 1; $i <= 10; $i++) {
$num_array[]=(crc32(genRandomURL()) % 4)+1;
}
print "<pre>";
print_r(array_count_values($num_array));
print "</pre>";
function genRandomURL($length = 10) {
$characters = '0123456789abcdefghijklmnopqrstuvwxyz';
$string = "";
for ($p = 0; $p < $length; $p++) {
$string .= $characters[mt_rand(0, strlen($characters))];
}
return "http://www.website.com/dir/dir2/dir3/".$string.".png";
}
?>
Results:
Array
(
[3] => 2489
[1] => 2503
[2] => 2552
[4] => 2456
)
Array
(
[1] => 5
[2] => 1
[3] => 3
[4] => 1
)

Answer 1

How about creating a hash of the name, getting the last two bits of that hash, then finally converting them back into a decimal number. Should return the same value so long as your name doesn't change.
function img_id($string){
$hash = md5($string); // create hash
$bin_hash = base_convert($hash, 16, 2); // convert to binary
$last_bits = substr($bin_hash, -2); // get last two bits
$img_int = bindec($last_bits)+1; // turn bits to integer, and + 1
return $img_int; // will be number from 1 to 4
}
$picture = 'picture.png';
$cdn_id = img_id($picture);
$url = "cdn{$cdn_id}.website.com/{$picture}";
If your name might change then you could also look at doing a hash of the actual file contents.