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

$c->insert( [ '_id' => 'circ3', 'name' => 'Humberto', 'performers' => 12 ] );
$c->update(
    [ 'name' => 'Humberto' ],            // query
    [ '$inc' => [ 'performers' => 4 ] ]  // update spec
);
var_dump( $c->findOne( [ 'name' => 'Humberto' ] ) );
?>