(PECL mongo >=0.9.0)
MongoCollection::createDBRef — Creates a database reference
document_or_id
If an array or object is given, its _id field will be used as the reference ID. If a MongoId or scalar is given, it will be used as the reference ID.
Returns a database reference array.
If an array without an _id field was provided as the
document_or_id parameter, NULL will be returned.
Example #1 MongoCollection::createDBRef() example
<?php
$songs = $db->songs;
$playlists = $db->playlists;
// create a reference to a song
$manamana = $songs->findOne(array('title' => 'Ma na ma na'));
$refToSong = $songs->createDBRef($manamana);
// add the reference to my playlist
$playlists->update(array('username' => 'me'), array('$push' => array('songlist' => $refToSong)));
?>