PHP mongodb - list all keys in a collection

gistfile1.php

$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));
// assuming first key is MongoID so skipping it
foreach (array_slice($keys,1) as $key => $value) {
    echo "<th>" . $value . "</th>";
}