<?php

function generate_auto_increment_id($namespace, array $option = array())
{
    $option += array(
        'init' => 1,
        'step' => 1,
    );

    $instance = new Mongo();

    $instance = $instance->selectCollection('_seq', 'seq');

    $seq = $instance->db->command(array(
        'findAndModify' => 'seq',
        'query'         => array('_id' => $namespace),
        'update'        => array('$inc' => array('id' => $option['step'])),
        'new'           => true,
    ));

    if (isset($seq['value']['id'])) {
        return $seq['value']['id'];
    }

    $instance->insert(array(
        '_id' => $namespace,
        'id'  => $option['init'],
    ));

    return $option['init'];
}

var_dump(generate_auto_increment_id('foo'));
var_dump(generate_auto_increment_id('bar', array('init' => 123)));

?>