array_push に問題があります。"Parse error: syntax error, unexpected T_DOUBLE_ARROW"
変数 $vars は、paypal の連想配列です。
array_push($vars,
'item_number'.$num => $id,
'item_name'.$num => $cart_item->name,
'amount_'.$num => $cart_item->discount_price,
'quantity_'.$num => $value);
$vars = array (
'cmd' => '_cart',
'charset' => 'utf-8',
'upload' => '1',
'currency_code' => 'HKD',
'amount' => $_SESSION['total'],
'custom' => $user_data->id)