How to setup user mapping for EXIM4?

Question

PHP

Sorry in advance, most likely the problem has been chewed up, but there was no suitable answer to my googling.
The PHP script should send mail with something like this:
mail("[email protected]", "subject", "my message", "From: [email protected]\r\n");
EXIM is configured in smarthost mode and works fine. But this PHP construction takes the current user. These are www-data, developer, root and other comrades, which are not provided on my mail server.
How to map www-data to, for example, sales, lovers, kitty, or something else nice for the user? Where to dig your nose?
Thank you.