Code snippet:
=====================
$wConcern=array("0"=>"Fire and forget",
                "1"=>"Write acknowledged by the primary",
                "2"=>"Write confirmed in 2 secondaries",
                "3"=>"Write confirmed in 3 secondaries",
                "majority"=>"Write confirmed in the majority of secondaries");

# loop through write concern options
foreach ($wConcern as $wc=>$desc){

  print "Write concern: $wc : $desc \n";

  $randomNumber=rand(10000000, 10000000000);
  $doc = array (
        "className" => "nosql.test",
        "source" => $randomNumber . "_nosql.test",
        "domain" => $randomNumber . "_nosql.test",
        "username" => $randomNumber . "_nosql.test",
        "email" => "nosql.test@gmail.com",
        "externalId" => "9999999",
        "policySlug" => "NEW_POLICY",
        "passwordHash" => "xxxxxxxxxxxxxx",
        "active" => true,
        "identityId" => "888888888",
        );
  $startTime = microtime(true);
  $thisColl->insert($doc, array("w" => $wc));
  $endTime = microtime(true);
  $timeDiff = number_format($endTime - $startTime, 10);

  print "Cursor return: $timeDiff  \n \n";
}
=====================