If I understand correctly, you would like your customers to be directed to different websites depending upon their IP address. If so, a simple PHP function should be fine.
The main question I have is how would you like to maintain/edit the IP/URL data. Is this a very large list of customers & URLs that would require a database back end, or would a text file suffice?
Also, do you require wildcard matching for the IP addresses, (i.e. 123.123.123.*), or will the full address always be known?
I would very much like to write this pushURL() function for you. Please do not hesitate to contact me with any questions.
Yours,
Mark