Hi Lexi,
which Debian version?
For a quick smock test, I've just started a docker container running PHP 7.2.8 on stretch as root and from a bash "console":
# cat /etc/debian_version
9.5
# getent passwd www-data
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
# echo "<?php echo posix_getpwuid(posix_geteuid())['name'].\"\\n\";" > /tmp/test.php
# cat /tmp/test.php
<?php echo posix_getpwuid(posix_geteuid())['name']."\n";
# php /tmp/test.php
root
# apt-get update
# apt-get install sudo
# which php
/usr/local/bin/php
# sudo -u www-data /usr/local/bin/php /tmp/test.php
www-data
HTH,
Matteo