Require very urgent Help about Security (pki, digital signature) along with PHP

Hi, I want to develop one payment site, where people will pay their tax. The payment is made through the payment gateway. But database entries are on local machine. From these database entries I am generating reports which need to encrypt and send it through mail or keep for download (which one is better?). The client will open / download it on his local machine and will decrypt it for reading. These reports are very sensitive, so I want high end security (I am out of USA). I am thinking of using pki (public key infrastructure). One key will be used to encrypt and another key to decrypt. Server will encrypt it and client will decrypt it. I hope there will be some companies who will be providing ready made software for that. Whether anybody can recommend about it? Also require more detail information about it. I also want to access the things using PHP / ASP / any web based language. How to encrypt the reports using PHP / ASP. Please help me as this is very urgent. regards, manisha