>Anyone would know of a good encryption code to encrypt a string but to obtain digits only?

Just a thought... one-way or two-way encryption.... if one-way, you could use the hashing algorithm. However, that would put in special characters, which could be converted down to bit format (8 bits per single char), then you'd have only 1+0... Then you could know that you are expecting each 8 bits = a single character... whatever that character was, then go from there...

Whats the reason for digits only as opposed to alpha/numeric.