Generate an ID string that form 24 hexidecimal characters, just like a MongoId string.

generate_id_hex.php

<?php
/** 
 *           File:  generate_id_hex.php
 *         Author:  Alvan
 *       Modifier:  Alvan
 *       Modified:  2014-03-28
 */
error_reporting(E_ALL);
ini_set('display_errors', 'on');
date_default_timezone_set('UTC');
 
/**
 * Generate an ID, constructed using:
 *   a 4-byte value representing the seconds since the Unix epoch,
 *   a 3-byte machine identifier,
 *   a 2-byte process id, and
 *   a 3-byte counter, starting with a random value.
 * Just like a MongoId string.
 *
 * @link http://www.luokr.com/p/16
 * @link http://docs.mongodb.org/manual/reference/object-id/
 * @return string 24 hexidecimal characters
 */
function generate_id_hex()
{
    static $i = 0;
    $i OR $i = mt_rand(1, 0x7FFFFF);
 
    return sprintf("%08x%06x%04x%06x",
        /* 4-byte value representing the seconds since the Unix epoch. */
        time() & 0xFFFFFFFF,
 
        /* 3-byte machine identifier.
         *
         * On windows, the max length is 256. Linux doesn't have a limit, but it
         * will fill in the first 256 chars of hostname even if the actual
         * hostname is longer.
         *
         * From the GNU manual:
         * gethostname stores the beginning of the host name in name even if the
         * host name won't entirely fit. For some purposes, a truncated host name
         * is good enough. If it is, you can ignore the error code.
         *
         * crc32 will be better than Times33. */
        crc32(substr((string)gethostname(), 0, 256)) >> 8 & 0xFFFFFF,
 
        /* 2-byte process id. */
        getmypid() & 0xFFFF,
 
        /* 3-byte counter, starting with a random value. */
        $i = $i > 0xFFFFFE ? 1 : $i + 1
    );
}
 

/**
 * test:
 *     php generate_id_hex.php | grep -i "Duplicated!" | wc -l
 */
$table = array();
for ($i = 0; $i < 100000; $i++)
{
    $_ = generate_id_hex();
    var_dump($_);
 
    if (isset($table[$_]))
    {
        $table[$_] += 1;
        var_dump('Duplicated!');
    }
    else
    {
        $table[$_] = 1;
    }
}
// End of file : generate_id_hex.php