Mongo inspired unique and sortable id generation

ObjectId.php

<?php

class ObjectId
{
    /** @var string */
    private $value;

    /**
     * @param string $value
     */
    public function __construct($value = null)
    {
        $this->value = $value ? : static::generate();
    }

    /**
     * @return string
     */
    public function valueOf()
    {
        return $this->value;
    }

    /**
     * @return string
     */
    public function getTimestamp()
    {
        $sec = hexdec(substr($this->value, 0, 8));
        $usec = hexdec(substr($this->value, 8, 7));
        return $sec . '.' . $usec;
    }

    /**
     * @return int
     */
    public function compareTo($other)
    {
        $ts1 = $this->getTimestamp();
        $ts2 = $other->getTimestamp();

        if ($ts1 > $ts2) {
            return 1;
        }

        if ($ts1 < $ts2) {
            return -1;
        }

        return 0;
    }

    /**
     * @return string
     */
    public function __toString()
    {
        return sprintf('%s(%s)', get_class($this), $this->valueOf());
    }

    /**
     * @return string
     */
    public static function generate()
    {
        list($usec, $sec) = explode(' ', microtime());
        return sprintf('%08x%07x%05x', $sec, substr($usec, 2), mt_rand(0, 0xfffff));
    }
}