fieldToLower.php

<?php

$host = null;

function db()
{
	global $host;
	$m = new MongoClient("mongodb://$host:27017");
	return $m;
}



function fieldToLower($db, $collection, $field)
{
	$m = db();
	$d = $m->selectDB($db);

	$c = $d->selectCollection($collection);
	
	foreach ($c->find() as $key => $value) 
	{
		if(isset($value[$field]))
		{
			$old = $value[$field];
			$c->update(['_id' => new MongoId($value['_id'])], ['$set' => [$field => strtolower($value[$field])]]);
			$new = strtolower($value[$field]);

			print("Changed $old ========> $new \n");
		}
	}
}

function getInput($field)
{
	print("Enter $field Name :: ");

	$handle = fopen ("php://stdin","r");
	$line = fgets($handle);

	if(trim($line) != '')
		return trim($line);

	return null;
}

function connectToHost()
{
	global $host;

	$host = getInput('Host IP');
	
	if(!is_null($host))
	{
		try
		{
			$m = db();
			return true;
		}
		catch(MongoConnectionException $ex)
		{
			print("\n");
			print("Unable to connect to datbase. \nGood Bye!");
			return false;
		}
	}

	print("Sorry! Host cannot be null. \nGood Bye!");
	return false;
}


function init()
{

	if(!connectToHost())
		return;

	$db = getInput('Database');
	
	if(!is_null($db))
	{
		$collection = getInput('Collection');
		
		if(!is_null($collection))
		{
			$field = getInput('Field');

			fieldToLower($db, $collection, $field);
		}
	}
}

init();

this is a simple command line script for mongodb manager,
you can connect to any host, select collection, and set the field to lower case.