mysqli bug

<?php
    define("TESTINPUT", '{"username":"Your db username","password":"Your db password","db":"test","ingredient":{"Ingredient":{"IngredientId":"68"},"Amount":20000}}');

    //$input = json_decode(file_get_contents('php://input'));
    $input = json_decode(TESTINPUT);

    $success = true;
    $db = new mysqli('localhost', $input->username, $input->password, $input->db);

    //Init
    $db->autocommit(false);
    $stmt = $db->prepare("INSERT INTO INGREDIENT_BATCHES (IngredientId, StockLevel) VALUES (?, ?)");
    $plab = $db->prepare("INSERT INTO PENDING_LABELS (Barcode) VALUES (?)");
    $ingredient = $input->ingredient;

    //Bind param
    $stmt->bind_param("ii", $ingredient->Ingredient->IngredientId, $ingredient->StockLevel);

    //Execute
    $stmt->execute();

    // This code works
    //$temp = $db->insert_id;
    //$plab->bind_param("i", $temp);

    // This code doesn't
    $plab->bind_param("i", $db->insert_id);
    $plab->execute();
    if ($plab->error != "") {
        $success = false;
        $errmsg = "Error executing query: " . $plab->error;
    }

    //Commit/Rollback
    if ($success) {
        echo "Able to commit";
        //$db->commit();
    } else {
        $db->rollback();
        echo $errmsg;
    }
?>