Okay... A few things about this that I must point out:
1. Are you using a regular HTTP web request? If so, your traffic can easily be sniffed; someone could either bruteforce the login URL, they could bypass it all in all with .NET reversing, they could sniff traffic to find out what you are sending and receive that back, etc. Anything like this isn't a safe login system, and can easily be bypassed.
2. Maybe make an email verification system? It's quite simple to be honest, and it will save you time with people that tend to like to spam emails. Also, I would suggest looking into SSL with an HTTP web request, so they can't get your POST info when you make a new account.
3. To be honest, this is just a few simple If statements. Make it a bit more dynamic

Have some protection in there as well that doesn't render your application vulnerable to particular attacks.
4. Make sure that in your PHP script, you are cleaning POST data...This may not seem important to you, but it's really a big deal.