Update MongoDB field using value of another field in PHP - php

Question

In MongoDB PHP, is it possible to update the value of a field using the value from another field?
The below Mongo Shell Command is working fine. Value of a field "product_name" is updating to the field "product_name_copy".
db.products.update(
{},
[{ $set: {
"product_name_copy": "$product_name"
}}],
{ multi: true }
)
But, While Am using this code in PHP,
PHP code :
$where = array();
$update = array('$set' => array("product_name_copy" => "$product_name"));
$options = array("multi" => true);
$bulkWrite = new MongoDB\Driver\BulkWrite;
$bulkWrite->update($where,$update,$options);
$updated = $mongo->executeBulkWrite("$db_name.products", $bulkWrite);
Am getting this error
Notice: Undefined variable: product_name in
C:\xampp\htdocs\mongo\mongo-update.php on line 10
If I used single quotes instead of double quotes,
$update = array('$set' => array("product_name_copy" => '$product_name'));
the same value $product_name is updating to the field "product_name_copy" like below.
{
"_id" : ObjectId("602a291f0406011059006c15"),
"product_id" : 15,
"product_name" : "Test Product",
"product_name_copy" : "$product_name"
}
Anyone know the solution for this problem.?

Answer 1

Finally I found the solution.
We need to put '$set' array into another one array for field value update.
PHP Code :
$where = array();
$update = array(array('$set' => array("product_name_copy" => '$product_name')));
$options = array("multi" => true);
$bulkWrite = new MongoDB\Driver\BulkWrite;
$bulkWrite->update($where,$update,$options);
$updated = $mongo->executeBulkWrite("$db_name.products", $bulkWrite);
Output of the above code is
{
"_id" : ObjectId("602a291f0406011059006c15"),
"product_id" : 15,
"product_name" : "Test Product",
"product_name_copy" : "Test Product"
}
This is what I exactly wanted.