I need to run some process (ordinary php-cli script) as some user. In
my simple.php I am launching some program for X (e.g. firefox).

startproc -u someuser /home/someuser/simple.php

1) as root: simple.php is running, but firefox is not
2) as someuser: everything is ok, firefox is running
3) even if I run ./simple.php, everything is ok, firefox is running

What is wrong?

Perhaps I need to set some environment variables? (DISPLAY and
XAUTHORITY I have already tryied, the same result)

With combination like follows, things work fine:
# as user root
startproc /home/someuser/simple.php
# in simple.php is:
su someuser -m -c "firefox"

Thank you for help.