"message": "Illegal string offset 'product_id'", In Laravel RESTfull API - json

Question

Am developing Laravel RESTful API for saving array of Json object into DB using php associative array and this is the error am getting.;
"message": "Illegal string offset 'product_id'",
"exception": "ErrorException",
...
This is the array of JSON object am trying to save in Mysql DB
"userId": "1",
"userName": "Dan",
"userEmail": "dan#gmail.com"
"product": [
{
"product_id": "31",
"product_name": "Orange",
"product_price": "USD 0.5"
}
]
}
This is the php function for save Json object into DB.
$userId = $request -> input('userId');
$userName = $request ->input('userName');
$userEmail = $request ->input('userEmail');
$data = array(
"userId"=>$userId,
"userName"=>$userName,
"userEmail"=>$userEmail
);
$user = User::create($data);
if($order){
//Storing the received JSON in $json.
$json = file_get_contents('php://input');
// Decode the received JSON and store into $obj
$obj = json_decode($json,true);
foreach($obj as $product){
$product_id = $product['product_id'];
$product_name = $product['product_name'];
$product_price = $product['product_price'];
$product = array(
"product_id" => $product_id,
"product_name" => $product_name,
"product_price" => $product_price
);
DB::table('products')->insert($product);
}
if($product){
// On query success it will print below message.
$MSG = 'Data Successfully Submitted.' ;
// Converting the message into JSON format.
$json = json_encode($MSG);
// Echo the message.
echo $json ;
}
else{
echo 'Try Again';
}
Guys where am I making mistake. Thank you in advance!