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

$r = $c->find(
  // query
  [ 'population' => [ '$gte' => 12000000 ] ],
  // projection
  [ 'name' => 1, 'population' => 1, '_id' => 0 ]
);

foreach( $r as $res )
{
    echo $res['name'], ': ', $res['population'], "\n";
}