I want to create a simple login functionality in WP7 app using remote MySQL database using PHP as back-end. I have never used this in C#, so I don't know how to do this.
You can use WebClient or HttpWebRequest class to make a web request and get the response.
Here is a sample code on how to make a request and get response
WebClient client = new WebClient();
client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
client.DownloadStringAsync(new Uri("http://someurl", UriKind.Absolute));
And the asynchronous response handler is here
void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
var response= e.Result; // Response obtained from the web service
}
The above example works for any web service(it may be PHP or jsp or asp etc).
All that you need to do is to make a proper request and handling the response