Web services was always easy to install on macOS Server and it’s no different on a Synology. To do so, open Package Manager from the home screen.
Click All in the sidebar and enter web into the search box.
Click Web Station.
Click Install. This installs a few dependencies. Click Open once the install is finished.
Click General Settings. Note that the default web server is Nginx. You can install Apache and then Apache will be available in the HTTP back-end server list. If you’ll be using a different service (Apache) then do the switch before you proceed. 
Otherwise (or after you switch to Apache), click on Virtual Host.
Click on Create.
Click into the hostname field and provide the name of the site. The ports can stay as are unless you’d like to customize the port that a site runs on. Then select a document root. This is where you’ll place your index.html or index.php file that sits at the root of a site.
Select the back-end server (e.g. Nginx or Apache 2.4) and then the PHP Profile (I usually stick with the default profile unless I’m using a method in PHP that’s unsupported in 7.x).
Click OK. And that’s it. Put your web directory into the document root, and viola – you have a new web server.