<?php
/* Custom document class that stores a UTCDateTime and timezone and also
 * implements the UTCDateTime interface for portability. */
class LocalDateTime implements \MongoDB\BSON\Persistable, \MongoDB\BSON\UTCDateTimeInterface
{
    private $utc;
    private $tz;
    public function __construct($milliseconds = null, \DateTimeZone $timezone = null)
    {
        $this->utc = new \MongoDB\BSON\UTCDateTime($milliseconds);
        if ($timezone === null) {
            $timezone = new \DateTimeZone(date_default_timezone_get());
        }
        $this->tz = $timezone;
    }
    public function bsonSerialize()
    {
        return [
            'utc' => $this->utc,
            'tz' => $this->tz->getName(),
        ];
    }
    public function bsonUnserialize(array $data)
    {
        if ( ! isset($data['utc']) || ! $data['utc'] instanceof \MongoDB\BSON\UTCDateTime) {
            throw new Exception('Expected "utc" field to be a UTCDateTime');
        }
        if ( ! isset($data['tz']) || ! is_string($data['tz'])) {
            throw new Exception('Expected "tz" field to be a string');
        }
        $this->utc = $data['utc'];
        $this->tz = new \DateTimeZone($data['tz']);
    }
    public function toDateTime()
    {
        return $this->utc->toDateTime()->setTimezone($this->tz);
    }
    public function __toString()
    {
        return (string) $this->utc;
    }
}
$bson = MongoDB\BSON\fromPHP(['date' => new LocalDateTime]);
$document = MongoDB\BSON\toPHP($bson);
var_dump($document);
var_dump($document->date->toDateTime());