"message": "Illegal string offset 'product_id'", "exception": "ErrorException",

Question

$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'];
          $data=array("product_id"=>$product_id);
          DB::table('order_products')->insert($data); 
        }

Answer 1

What's inside the $product in your foreach-loop?

Example:

If its array( product_id => 1 ), the result of $product['product_id'] would be 1.

But if there's an error and $product is for example {"product_id":1} (a string instead of array), the result of $product['product_id'] would be an error.

Possible Fix:

You can try $product = json_decode($product, true); inside your foreach-loop, maybe $product isn't decoded and still a string, so this would fix your problem.