When posting PHP code, please use the board's [noparse]
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[/noparse] bbcode tags as they make your code much easier to read and analyze.
As for your problem, you simply need to ad another header to the message in the form of: "BCC: <email>".
The first parameter of the mail() call is really just a shortcut to adding a 'To: <email>' header to the e-mail, since an e-mail really only consists of a header and body.
Also note that you should add a 'Sender' header that points to an e-mail address on your domain (e.g. the same domain from which the e-mails are being sent), unless such a header is already being added automatically by your MTA. Otherwise, you run the risk of being labeled as an open relay and automatically blacklisted from many e-mail servers.
Finally, note that you should never use any user-supplied data in the headers area without first sanitizing it; otherwise, your code will be vulnerable to header injection attacks. For validating e-mail addresses, for example, you could use [man]filter_var/man with the FILTER_VALIDATE_EMAIL option.