Synala
Synala is a free, open sourced PHP system that you install on your server, enabling you to easily and securely accept Bitcoin payments online. No more need to signup for a merchant service like BitPay, as Synala allows you to easily become your own payment processor, with absolutely no fees, while you remain in 100% control over your own funds.
Fully supports multiple BIP32 wallets, multisignature transactions, offline signing, user registration, products, invoices, and more. No private keys are stored online at any time, making your funds just as secure as cold / offline storage.