$m = new Mongo();
$db = $m->test;
$collection = $db->david;
$cursor = $collection->find();
$array = iterator_to_array($cursor);
$keys = array();
foreach ($array as $k=>$v) {
        foreach ($v as $a=>$b) {
                $keys[] = $a;
        }
}
$keys = array_values(array_unique($keys));

Takes the collection 'david' and gives a unique array of the keys used.

List of Keys Used in MongoDB Collection

php, array

PHP