if you want to set uid by username rather than uid, try combining it with posix_getpwnam(),
<?php
posix_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,
<?php
posix_seteuid(0);
posix_setegid(0);
?>