new.php

<?php
function get_new_image_name($dir, $id, $limit_num = PHP_INT_MAX)
{
    $cache = [];
    foreach(glob($dir . sprintf('/%02u-*.*', $id), GLOB_MARK) as $name)
    {
        if(is_dir($name))
        {
            continue;
        }

        if(preg_match('/^\d+\-(?P<counter>\d+)\_/', basename($name), $ta))
        {
            $cache[(int)$ta['counter']] = true;
        }
    }

    if(count($cache) >= $limit_num)
        throw new RuntimeException("Too many files already uploaded under this id.", 409);

    $counter = 1;
    while(isset($cache[$counter]))
    {
        if($counter >= $limit_num)
            throw new RuntimeException("Too many files already uploaded under this id.", 409);

        $counter++;
    }

    // name = id-index_[sml|med|big].ext
    return sprintf('%02u-%02u_', $id, $counter);
}

old.php

<?php
function get_new_image_name($dir, $id, $limit_num = null)
{
    $files = get_file_list($dir); // reads entire dir into an array
    $counter = 0;
    // name = id-index_[sml|med|big].ext
    while(($name = sprintf('%02d', $id) . '-' . sprintf('%02d', ++$counter) . '_'))
    {
        $notFound = true;
        foreach ($files as $file)
        {
            if ((strstr($file, $name) !== false))
            {
                $notFound = false;
                break;
            }
        }

        // Limit number of uploaded files
        if ($notFound || ($limit_num !== null && $counter >= $limit_num))
        {
            return $name;
        }
    }

}