emails form nobody

Hi

I set up my postfix to work as a relay to send emails form my php scripts. And its working. Thank
you very much for the help form this newsgroup.

My problem now is that all the emails are coming from nobody@example.com.
What I need to do to make them coming form something like ralph@something.com

If i change the line mydomain = something.com its just chancing the domain on the send emails.
what i need to do to change user from which the emails are coming.

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Ralph