NGINX is much more then just a server like Apache, that's probably why it gained so much market share over the few last years if you trust the Netcraft server market share statistics.
But as i said NGINX is more then just a server. The powerful configuration mechanism lets you setup NGINX to be a proxy server for one or multiple apache servers. You can use it as load balancer for multiple node.js instances or "just" use it as normal server to host your php website. If it's a php website you want to host.