Change user which executes script called by postfix (mail pipe)

hi
I have a executable php-script that's called from postfix with a virtual alias:

virutal:
mail-addr..(is blocked by forum ) test

aliases:
test "| /test/test.php"

this works great....
the executing user is "nobody", and if I write a file with the script, the file's owner is "nobody".

how can I change this user to "www-data"?
I want to save incoming mails and make them available to download via http.

Greets and thanks for your help!
Puschkin