Basic encode an int into a short hash.

hash.php

<?php
/**
 * Encode an int into a short hash.
 * 
 * @param int $id
 * @param string|int $seed - Lock alphabet shuffle, or leave blank for random
 * @return string
 */
function id_encode(int $id, $seed = null) {
    $alphabet = str_split('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890');

    if ($seed !== null) {
        @mt_srand($seed);
        for ($i = count($alphabet)-1; $i > 0; $i--) {
            $a = @mt_rand(0, $i);
            $t = $alphabet[$i];
            $alphabet[$i] = $alphabet[$a];
            $alphabet[$a] = $t;
        }
    } else {
        shuffle($alphabet);
    }

    $value = base_convert($id+"1e3", 10, 36);

    $out = '';
    foreach (str_split($value) as $key => $value) {
        $out .= $alphabet[$key].$value;
    }

    return str_rot13($out);
}

/**
 * Decodes hash into int
 * 
 * @param string $hash
 * @return int
 */
function id_decode($str) {
    $str = str_rot13($str);

    $out = '';
    foreach (str_split($str) as $key => $value) {
        if ($key % 2 != 0) {
            $out .= $value;
        }
    }
    return base_convert($out, 36, 10)-"1e3";
}