PHP Mongo ObjectId generator

object_id_generator.php

<?php
function generate_object_id() {
    $timestamp = str_pad(dechex(time()), 8, "0", STR_PAD_LEFT);
    $machine = str_pad(dechex(rand(0, 16777215)), 6, "0", STR_PAD_LEFT);
    $pid = str_pad(dechex(rand(0, 32767)), 4, "0", STR_PAD_LEFT);
    $increment = rand(0, 16777215);
    
    $increment++;
    
    if ($increment > 0xffffff) {
        $increment = 0;
    }
    
    $increment = str_pad(dechex($increment), 6, "0", STR_PAD_LEFT);
    
    return $timestamp . $machine . $pid . $increment;
}

object_id_generator_opt.php

<?php
function generate_object_id() {
    return dechex(time()) . preg_replace_callback('/[x]/', function($match) {
        return dechex((rand() / getrandmax()) * 16);
    }, 'xxxxxxxxxxxxxxxx');
}