Add to a Simple XML Object

I need someone to quickly solve this problem and tell me whats being done wrong. First solution gets paid. I need to add additional products to the $products object using the $fields array to set the key/values. This job will pay $15. It should be very quick for someone who knows just a little more than me. When using the code below to add a product to the $products SimpleXML object I get an error Warning: Illegal offset type in file and can't figure out what is wrong. $fields = array( $o->order_id => 'order_id', $o->order_address_id => 'order_address_id', $o->price_ex_tax => 'price_ex_tax' , 'DISCS' => 'sku', '1' => 'quantity' ); $products->addChild('product'); array_walk_recursive($fields, array ($products, 'addChild')); HERE IS MY SIMPLEXML OBJECT using print_r for $products: SimpleXMLElement Object ( [product] => Array ( [0] => SimpleXMLElement Object ( [id] => 23 [order_id] => 110 [product_id] => 63477 [order_address_id] => 11 [name] => Dead Sea Treasures Foot Cream [sku] => 703 [type] => physical [base_price] => 20.0000 [price_ex_tax] => 20.0000 [price_inc_tax] => 20.0000 [price_tax] => 0.0000 [base_total] => 40.0000 [total_ex_tax] => 40.0000 [total_inc_tax] => 40.0000 [total_tax] => 0.0000 [weight] => 0.5 [quantity] => 2 [base_cost_price] => 0.0000 [cost_price_inc_tax] => 0.0000 [cost_price_ex_tax] => 0.0000 [cost_price_tax] => 0.0000 [is_refunded] => false [refund_amount] => 0.0000 [return_id] => 0 [wrapping_name] => SimpleXMLElement Object ( )