<?php
$m = new MongoClient();
$c = $m->demo->cities;

$res = $c->aggregate( [ 
    [ '$match' => [ 'name' => 'Atlanta' ] ],
    [ '$unwind' => '$alternatenames' ],
    [ '$project' => [
        'name' => '$alternatenames',
        'meta' => [
            'elevation' => '$dem',
            'population' => 1,
            'timezone' => 1,
        ],
        'location' => 1,
        '_id' => 0,
    ] ],
    [ '$sort' => [ 'name' => 1 ] ]
] );

var_dump( $res );
?>