Simple Hash Function to Create "Tiny URL" Slug

<?php
public function makehash($num)
{
    $alphabet = 'abcdefghijklmnopqrstuvwxyz_0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $base_count = strlen($alphabet);

    $encoded = '';

    while ($num >= $base_count)
    {
        $div = $num/$base_count;
        $mod = ($num-($base_count*intval($div)));
        $encoded .= $alphabet[$mod];
        $num = intval($div);
    }

    if ($num) $encoded .= $alphabet[$num];

    return $encoded;
}