(PECL mongo >=1.3.0)
MongoDB::getCollectionNames — Get all collections from this database
$includeSystemCollections = false
  ] )Returns an array of all the collection names for the given database.
includeSystemCollectionsInclude system collections.
Returns the names of the all the collections in the database as an array.
Example #1 MongoDB::getCollectionNames() example
<?php
$m = new Mongo;
$collections = $m->selectDB("demo")->getCollectionNames();
var_dump($collections);
?>
The above example will output something similar to:
array(2) {
  [0]=>
  string(9) "addresses"
  [1]=>
  string(5) "users"
}