posix_geteuid

if you for some reason need the euid without depending on php-posix being available, try

<?php
function geteuid_without_posix_dependency(): int
{
    try {
        // this is faster if available
        return \posix_geteuid();
    } catch (\Throwable $ex) {
        // php-posix not available.. fallback to hack
        $t = tmpfile();
        $ret = fstat($t)["uid"];
        fclose($t);
        return $ret;
    }
}

Please note the example code shown above is invalid and will fail, since UID 10001 cannot use posix_seteuid to change its UID to 10000