Summary of answers:
Don't do it. The legal and financial implications will be disastrous. Look for established third party solutions or hire an expert. Never store any sensitive information on a shared server. Research for the most appropriate encryption mechanism.
Ben doğrudan mevduat için db (+ hesap numarası yönlendirme) müvekkillerinin banka bilgi depolamak için gereken bir müşteri için bir web sitesi buiding duyuyorum. İşte bazı özellikleri şunlardır:
1) The website will initially be on a shared hosting server (this is my first concern).
2) I am using PHP/MySQL.
3) I plan on using mcrypt.
4) The key will be located outside the web root.
Bana düşüncelerinizi bildirin. Mümkünse, ACH işleme bazı kaynaklar bana verin.
Teşekkürler!
EDIT: I expected such response as I am terrified of security issues out there also. I have expressed my concern to my customer and this will be a good support.
EDIT 2: Will walk away from this. Was not happy with the idea in the first place! Will investigate PayPal's Mass Mailer API.