There is nothing wrong with that. You are simply using a RESTful API to access the MySQL data, thus isolating the database details.
If one day you are tired of PHP, you can easily switch to Ruby, Python or Whatever for that part without even touching the node.js. If your logic is already written in PHP (you are upgrading an old app), it make even more sense as you can reuse what has already been tested and debugged. A lot of folks are advocating for that kind of separation between systems. Just look at all the SOA (Service Oriented Architecture) buzz.
Where I work we are using this very architecture in a project (though in this case its an ASP.NET MVC Website calling a Java EE app) and it served us very well. With the event model of node.js, its even better since you won't block waiting for the PHP.
But of course, there are some drawback
- Performance overhead
- Architecture is more complicated
- You now work with two language instead of only one (though javascript and PHP are so often used together that I don't think it's really is a problem in this case)
So you need to ask yourself if your problem really need that solution. But in a lot of case the answer may be yes. Just don't forget the virtue of Keeping It Simple and Stupid (the KISS principle)