How does changing file permission my site's php file (in /var/www/html) actually work? [duplicate]

I really don't know to phrase the question I'm having a really descriptive way so maybe it's best for you just to ignore the question up there for it can be misleading.

So I recently learned that by setting permission in my index.php or whatever.php file in /var/www/html by chmod 600 index.php, people who request that index.php file will get a 403 forbidden. That's happening because "600" means "other users" (users who don't belong to the group or are not the file's owner) have 0 permission. But how what user the web client is exactly?