在MongoDb php驱动程序中将JSON插入变量

i convert json file to array but it give me error in foreach invalid argument

<?php
require 'vendor/autoload.php'; 
$client = new MongoDB\Client;
$test = $client->test;
$createCollection = $test->restaurants;
$string = file_get_contents("D:\Database\primer-dataset.json");
$json_a = json_decode($string, true);

foreach ($json_a as  $id => $item)
{
    $createCollection->insert( $item);


?>

Since it says invalid argument, $string is probably not valid JSON. Check the contents of $string and $json_a to see what exactly is happening.