Several packages can help you serve PHP through Node, most notably the node-php packages (there's two of them, completely unrelated to each other but generally does the same job).
Here's a sample of Node serving WordPress using mkschreder's node-php:
var express = require('express');
var php = require("node-php");
var path = require("path");
var app = express();
app.use("/", php.cgi("/path/to/wordpress"));
app.listen(9090);
console.log("Server listening!");
Ideally though, you should stick to either just Node or PHP. Node operates server-side, so does PHP. Putting PHP on a Node project makes Node redundant, and vice versa.
Further reading: