In PrestaShop 1.6 (tested and confirmed working in v1.6.0.14) you can accomplish this by the following method.
Copy the file /classes/PaymentModule.php to /override/classes/PaymentModule.php.
Edit the file /override/classes/PaymentModule.php as follows.
At lines 337-341 is a code block that should read like this:
if (!result)
{
PrestaShopLogger::addLog('PaymentModule::validateOrder - Order cannot be created',
3, null, 'Cart', (int)$id_cart, true);
throw new PrestaShopException('Can\'t save Order');
}
Immediately after that code block, insert the following two lines of code:
$order->reference = str_pad($order->id, 9, '0', STR_PAD_LEFT);
$order->update();
Delete the file /cache/class_index.php so that Prestashop automatically re-creates this file taking into account the new override file.
Any existing records in your PrestaShop database can be updated to use a numerical reference manually using a tool such as phpMyAdmin.
I would imagine the steps would be very similar if not identical to these for PrestaShop v1.5 but at this time I have not tested this solution with PrestaShop v1.5. If someone finds this solution works on v1.5 perhaps they could confirm this in the comments. Thank you.
