You need to pass also \n
symbol to get your command work:
var spawn = require('child_process').spawn,
child = spawn('phantomjs');
child.stdin.setEncoding('utf-8');
child.stdout.pipe(process.stdout);
child.stdin.write("console.log('Hello from PhantomJS')\n");
child.stdin.end(); /// this call seems necessary, at least with plain node.js executable