PHP Mongodb aggregrate query error

Hi There,

I have a mongodb aggregate query in PHP that is throwing an error and I can't see what I've missed in the syntax.

Versions are
mongodb =  3.6
PHP  = 7.1
mongo PHP driver = 1.3.1-1


 The query in mongo shell works and looks like this:

db.products.aggregate(
	[
		{
			$match: {
			    download_Date : {'$gte' : 20180205 }
			}},
		{
			$sort: {
			download_Date: 1 }
		},
		{
			$group: {
			 _id: "$cw_product_id", batch: { $last : "$download_Date" }
	        }},
	]

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My code in PHP fails and  looks like

$command = new MongoDB\Driver\Command([
    'aggregate' => 'products',
    'pipeline' => [    
        ['$match' =>  [ 'download_Date' => ['$gte' => $batch ]]],
        ['$sort'  => [  
            'download_Date' => 1
        ]],
    ['$group' => [ 
        '_id' => [ '$cw_product_id', 'batch' => ['$last' => '$download_Date'] ],
        ]],   
    ],
        'allowDiskUse' => true, 
        'cursor' => new stdClass, 
    ]);

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The error I receive when running the php script is

           

PHP Fatal error:  Uncaught MongoDB\Driver\Exception\RuntimeException: Unrecognized expression '$last'

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I've pulled it apart and put it back together so many times, a fresh pair of eyes is needed to spot the obvious.  

Thanks in advance,

Rick