if you want to set uid by username rather than uid, try combining it with posix_getpwnam(),<?phpposix_seteuid (posix_getpwnam("www-data")["uid"]);posix_setegid(posix_getpwnam("www-data")["gid"]);?>note that posix_getpwnam() reads /etc/passwd on each invocation, if you need it to go fast, this should be faster (only read passwd once):<?php$posix_user_info=posix_getpwnam("www-data");posix_seteuid ($posix_user_info["uid"]);posix_setegid($posix_user_info["gid"]);?>and to go back to being root, just set them to 0,<?phpposix_seteuid(0);posix_setegid(0);?>