Web project sub-domain design

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PHP

Apache

I am working on a project that would be using subdomains( my domain wildcards ) to show clients data

For example
if i registered my domain as ( www.example.com ),
my clients would be using as ( clientdomain.example.com )


Now the biggest problem am having is

1. should i write my php scripts by reading the passed in domain name from the url using the get request as ( www.example.com?client=clientdomain) then rewrite the url to match that of a client as ( clientdomain.example.com )

and if that's the case what will happen if client requested ( clientdomain.example.com)

Anybody who can help me in this design approach for this application will help me alot.

Thanks developers.