MongoDB - PHP Group By Count - php

Question

I have this code :
$data=$collection1->aggregate(array( '$group'=> array('_id'=>$valoreScelto,'contatore'=>array('$sum'=>1))));
$valoreScelto is a valid field of document MongoDB, that i retry by FORM.
$valoreScelto = trim('$'.$campoSelezionato);
I obtain this error:
Fatal error: Call to a member function aggregate() on string

Answer 1

UPDATE:
The error says: You are trying to access the method "aggregate", in the string (Variable $collection1 have type - string).
You need to check $collection1 (for example var_dump). $collection1 must be Collection (or in mongo extension MongoCollection).
You can get a collection like this:
$yourConnectInDB = new Client(...); // or MongoClient(...);
$db = $yourConnectInDB->selectDatabase('YOUR DB NAME');
$collection1 = $db->selectCollection('YOUR COLLECTION NAME');
Also, in your code you want to use the aggregation like this:
$ops = array( // base array
array(
'$group' => array(
"_id" => $valoreScelto,
"contatore" => array('$sum'=>1),
)
),
// other pipeline
);
$data=$collection1->aggregate($ops);
Read this and this

Answer 2

I tried the above answer as well and it wasn't returning anything. After a lot of trying I figured out that I had missed a keyword without which the above query was not going to work. I am pasting the code below.
$ops = [
[
'$group' => [
"_id" => $valoreScelto,
"contatore" => ['$sum'=>1],
]
]
];
$data=$collection1->aggregate($ops)->toArray();