db.crimes.aggregate([
{ $match: {"CrimeLSOAName":/.*Bradford.*/} },
{
    $group: {
    "_id": "$CrimeType",
    "count": {
        $sum: 1
        }
    }
},
{
$sort: {
    "count": -1
}
},
{
    $limit: 10
}
])

$top10Crimes = array(
array('$match') => array('CrimeLSOAName:/.*Bradford.*/'),
array(('$group') => array('_id' => '$CrimeType',
                     'count' => '$sum: 1')),
array('$sort') => array('count' => '-1')
array('$limit' => '10')
);

$result = $collection->aggregate($top10Crimes);