shorten_urls.php

function crb_encode_short_ref_code($id) {
	// if (!is_int($id)) {
	// 	echo 'not an int';		
	// }

	$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
	if ($id == 0) {
		return $chars[0];
	} 

    $number = abs($id);

    if ($id < 0) {
    	echo "Can not encode for negative integers";
    }

    $string = '';
    $base = strlen($chars);

    while ($number > 0) {
        $string .= $chars[$number % $base];
        $number = (int) ($number / $base);
    }

    return strrev($string);
}

function crb_decode_short_ref_code($string) {
	$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';

	$stringLength = strlen($string);

    $baseLength = strlen($chars);
    $id = 0;

    for($i = 0; $i < $stringLength; $i++) {
        $pos = strpos($chars, $string[$i]);
        $id = ($id * $baseLength) + $pos;
    }

    return $id;
}